Industrial wood finishing

Industrial wood
finishing
L I V I N G
F O R
W O O D
Industrial wood
finishing
T I K K U R I L A
O Y
I N D U S T R Y
Industrial wood
finishing
Editorial staff
Kaj Fagerholm
Heidi Hirvelä
Minna Ihamäki-Laitinen
Erkki Keränen
Keijo Korhonen
Leena Manni-Rantanen
Arto Nummela
Anu Passinen
Publisher
Tikkurila Oy, Industry
Copyright © 2010 Tikkurila Oy
ISBN 978-952-5030-43-3
First edition
Layout
Keijo Korhonen
Tanja Peltola
Printed by
Tikkurila Oy
I N D U S T R I A L
W O O D
F I N I S H I N G
Table of contents
1. Basic information on paints and lacquers............................................................................................... 1
1.1 Binders.................................................................................................................................................. 1
1.1.1 Amino resins................................................................................................................................. 1
1.1.2 Alkyd resins.................................................................................................................................. 1
1.1.3 Epoxy resins . .............................................................................................................................. 1
1.1.4 Urethane resins (isocyanate-hardened) ..................................................................................... 2
1.1.5 Nitrocellulose................................................................................................................................ 2
1.1.6 Cellulose acetobutyrate (CAB)..................................................................................................... 2
1.1.7 UV-hardened acrylate resins........................................................................................................ 2
1.1.8 Water-borne binders, i.e. dispersions.......................................................................................... 2
1.2 Pigments and fillers............................................................................................................................... 2
1.3 Thinners................................................................................................................................................. 3
1.4 Additives................................................................................................................................................ 3
2. Different types of coatings and their application methods.................................................................... 4
2.1 Putties.................................................................................................................................................... 4
2.2 Stains..................................................................................................................................................... 4
2.3 Wood preservatives............................................................................................................................... 4
2.4 Lacquers............................................................................................................................................... 5
2.4.1 Sanding sealers........................................................................................................................... 5
2.4.2 Top lacquers................................................................................................................................. 5
2.5 Paints..................................................................................................................................................... 5
2.6 Hardeners.............................................................................................................................................. 5
2.7 Thinners................................................................................................................................................. 6
3. Wood structure and different types of wood............................................................................................ 7
3.1 Wood..................................................................................................................................................... 7
3.1.1 The structure of wood.................................................................................................................. 7
3.1.2 Branches...................................................................................................................................... 8
3.2 Properties of wood................................................................................................................................ 8
3.2.1 The living wood............................................................................................................................ 8
3.2.2 Humidity....................................................................................................................................... 8
3.2.3 Temperature................................................................................................................................. 8
3.2.4 Time.............................................................................................................................................. 8
3.3 Wood species........................................................................................................................................ 9
3.3.1 Pine.............................................................................................................................................. 9
3.3.2 Spruce.......................................................................................................................................... 9
3.3.3 Birch............................................................................................................................................. 9
3.3.4 Oak............................................................................................................................................. 10
3.3.5 Ash............................................................................................................................................. 10
3.3.6 Other wood species................................................................................................................... 10
3.4 Board materials................................................................................................................................... 11
3.4.1 Fibreboards................................................................................................................................ 11
3.4.2 MDF and HDF boards................................................................................................................ 11
3.4.3 Plywood...................................................................................................................................... 11
3.4.4 Chipboards................................................................................................................................ 12
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4. Special requirements of different base materials in finishing work ................................................... 13
4.1 MDF board........................................................................................................................................... 13
4.2 HDF board........................................................................................................................................... 13
4.3 Plywood............................................................................................................................................... 13
4.4 Solid wood........................................................................................................................................... 14
4.4.1 Pine and spruce......................................................................................................................... 14
4.4.2 Oak and ash............................................................................................................................... 14
4.4.3 Birch........................................................................................................................................... 14
4.4.4 Tropical hardwood...................................................................................................................... 14
5. Sanding..................................................................................................................................................... 15
5.1 Sanding methods................................................................................................................................ 15
5.2 Sanding belts...................................................................................................................................... 15
5.3 Instructions for sanding....................................................................................................................... 16
6. Finishing and coating methods and equipment.................................................................................... 18
6.1 Spraying and spraying equipment...................................................................................................... 18
6.1.1 Spraying booths......................................................................................................................... 18
6.1.2 Conventional spraying .............................................................................................................. 19
6.1.3 Airless spraying.......................................................................................................................... 19
6.1.4 Air-assisted airless spraying...................................................................................................... 20
6.1.5 Electrostatic centrifugal method................................................................................................ 20
6.1.6 Electrostatic spraying................................................................................................................. 20
6.1.7 Spraying with heated coating.................................................................................................... 21
6.1.8 High-volume conventional spraying (HVLP spraying)............................................................... 21
6.1.9 Two-component spraying........................................................................................................... 22
6.1.10 Automatic spraying.................................................................................................................. 22
6.1.11 Robot spraying......................................................................................................................... 23
6.2 Coating methods................................................................................................................................. 24
6.2.1 Curtain coating........................................................................................................................... 24
6.2.2 Vacuum coating......................................................................................................................... 24
6.2.3 Roller coating application.......................................................................................................... 25
6.2.4 Dipping....................................................................................................................................... 25
6.2.5 Drum-coating............................................................................................................................. 25
7. Finishing lines.......................................................................................................................................... 26
7.1 Window industry.................................................................................................................................. 26
7.2 Door industry....................................................................................................................................... 27
7.3 Parquet industry.................................................................................................................................. 28
7.4 Moulding industry................................................................................................................................ 29
7.5 Finishing sawn timber.......................................................................................................................... 29
7.6 Kitchen furniture industry.................................................................................................................... 30
7.7 Furniture industry................................................................................................................................. 30
7.8 Other types of wood industry.............................................................................................................. 31
8. Drying and hardening............................................................................................................................... 32
8.1 Air drying............................................................................................................................................. 32
8.2 Heat drying.......................................................................................................................................... 32
8.3 Drying and hardening......................................................................................................................... 32
8.4 Physical drying.................................................................................................................................... 32
8.5 Chemical drying.................................................................................................................................. 32
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8.6 Heat transfer........................................................................................................................................ 32
8.6.1 Conduction................................................................................................................................. 32
8.6.2 Convection................................................................................................................................. 32
8.6.3 Radiation.................................................................................................................................... 33
9. Drying equipment (ovens)....................................................................................................................... 34
9.1 Drying tunnels..................................................................................................................................... 34
9.2 Multilayer ovens................................................................................................................................... 34
9.3 IR ovens............................................................................................................................................... 35
9.4 Gas IR.................................................................................................................................................. 35
9.5 MOS-drying......................................................................................................................................... 35
9.6 UV ovens............................................................................................................................................. 36
9.7 UVITEC® curing................................................................................................................................... 36
9.8 Electron-beam dryers (EB dryers)....................................................................................................... 37
9.9 Dry-air equipment................................................................................................................................ 37
9.10 UV-LED curing................................................................................................................................... 37
10. Testing the finished paint surface......................................................................................................... 38
11. Paint standards for wooden surfaces................................................................................................... 39
12. Health and safety at work; Environmental protection......................................................................... 40
12.1 Health and safety within the paint industry....................................................................................... 40
12.2 Package markings............................................................................................................................. 40
12.2.1 Reactive products.................................................................................................................... 40
12.2.2 Products which are hazardous to health.................................................................................. 40
12.2.3 Products which are hazardous to the environment.................................................................. 40
12.3 Safety data sheets............................................................................................................................. 41
12.4 Fire safety.......................................................................................................................................... 41
12.5 Using paints...................................................................................................................................... 42
13. Objects to be coated and recommended paint combinations........................................................... 43
13.1 Furniture...................................................................................................................................... 43
13.2 Fixtures........................................................................................................................................ 43
13.3 Internal doors.............................................................................................................................. 44
13.4 Lamella parquets........................................................................................................................ 44
13.5 Windows and exterior doors....................................................................................................... 44
13.6 Mouldings, frames and panels.................................................................................................... 45
13.7 Exterior claddings....................................................................................................................... 45
13.8 Beams......................................................................................................................................... 46
13.9 Plywood....................................................................................................................................... 46
13.10 Fibreboard................................................................................................................................. 46
14. Coating costs and cost calculations.................................................................................................... 47
15. General instructions before finishing................................................................................................... 50
16. Some of the most common concepts in painting............................................................................... 52
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17. Tinting systems....................................................................................................................................... 55
18. Solving finishing problems.................................................................................................................... 57
18.1 Orange-peel surface......................................................................................................................... 57
18.2 Poor coverage................................................................................................................................... 57
18.3 Paint film breakage in the curtain coating......................................................................................... 57
18.4 Air bubbles during painting............................................................................................................... 58
18.5 Air bubbles during the drying phase................................................................................................ 58
18.6 Ineffective drying and soft paint surface........................................................................................... 58
18.7 Surface cracking (reticular)............................................................................................................... 59
18.8 Surface cracking (grain-aligned)...................................................................................................... 59
18.9 Sagging............................................................................................................................................. 59
18.10 Colour differences in painting......................................................................................................... 59
18.11 Colour differences in lacquering..................................................................................................... 60
18.12 Colour differences in staining.......................................................................................................... 60
18.13 Gloss variations............................................................................................................................... 60
18.14 Wrinkling (shrinkage linking)........................................................................................................... 61
18.15 Mat blotches.................................................................................................................................... 61
18.16 Stripes............................................................................................................................................. 61
18.17 Impurities and unwanted material on the paint surface.................................................................. 62
18.18 Craters (Repulsion)......................................................................................................................... 62
18.19 Poor adhesion................................................................................................................................. 62
18.20 Greasiness (Sweating).................................................................................................................... 63
18.21 Light blotches on the lacquer surface............................................................................................. 63
18.22 Rough surface................................................................................................................................. 63
19. Colours.................................................................................................................................................... 64
19.1. Colour is light.................................................................................................................................... 64
19.2. Colour made visible.......................................................................................................................... 65
19.3. Colour formation............................................................................................................................... 66
19.3.1 Additive colour formation......................................................................................................... 66
19.3.2 Subtractive colour formation.................................................................................................... 66
19.4. Defining and measuring colour........................................................................................................ 66
19.5. Differences in colour......................................................................................................................... 67
19.6. Metamerism...................................................................................................................................... 67
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1. Basic information on paints and
lacquers
H
ereafter, paints and lacquers will be referred to simply
as paint. Paints can be categorized according to
their various properties, such as the binders, diluents or
drying method in question. Among the various binding
agents are amino resins, alkyd resins, latexes i.e.
dispersions, epoxy resins and urethane resins. Based
on the type of thinner, paints can be divided into waterborne, solvent-borne and solvent-free (100 %) paints.
Furthermore, based on the drying method, they can
be divided into physical-drying paints, oxidative-drying
paints and reaction paints.
Paints and lacquers comprises several different raw
materials that can be divided into four groups: binders,
pigments and fillers, solvents and additives. All of these
have particular significance in terms of the properties of
the paints and lacquers in the tin, when applied, and as
a completed surface.
1.1 Binders
The purpose of a binder is to bind together the paint’s
other ingredients, and to make the paint adhere to its
substrate. Binders are either solid or liquid polymers.
Solid binders with a high viscose content are diluted to
the appropriate liquidity before the paint is manufactured.
A binder can also consist of tiny polymer particles in
water (dispersion).
The binder determines the paint’s resistance
properties and its adhesion capacity with respect to the
substrate. For this reason, the categorization of paints is
generally based on the type of binder.
The following is a description of the binder types
– and their drying times – most commonly used in the
wood industry.
themselves. Amino resins always contain formaldehyde,
which is released when the paint dries. The formaldehyde
released from the dried film can be recognized by its
pungent smell. When the paint dries, the formaldehyde
emission is reduced. Amino resins are too fragile to be
used as such, and are therefore combined with binders
that give the paint elasticity.
1.1.2 Alkyd resins
Alkyds are polyesters that contain fatty acids originating
in vegetable oils, such as linseed oil, soybean oil, castor
oil, or tall oil fatty acids. By varying the raw materials,
different types of alkyds can be manufactured that
are either solvent-borne or dispersed in water. Alkyds
containing drying oil (such as linseed oil or soybean oil)
or fatty acids from drying oils (such as tall oil fatty acid)
are used for air-drying paints.
While alkyd paints dry, solvents or water evaporate
and the binder reacts with the oxygen in the air. In
catalytic paints, alkyds manufactured from non-drying
oils or fatty acids are used. By changing the quantity
of the alkyd, the hardness of the paint can be adjusted.
Paints intended for indoor use are made harder than those
suitable for outdoor use. Alkyds reacts chemically with
amino resins, building up a product whose mechanical
properties, as well as chemical resistance, are excellent.
To some degree, alkyds of this type also react with the
oxygen in the air, resulting in an even higher degree of
chemical resistance.
Urethane alkyds are also oxidative-drying binders,
whose properties resemble those of alkyds. Their
abrasion and chemical resistance is slightly higher than
that of alkyds.
Water-borne alkyds are used in base paints intended
for outdoor use in particular.
1.1.1 Amino resins
Carbamide and melamine resins are the most commonly
used amino resins. Carbamide resins dry quickly, but
their chemical resistance is not as good as that of
melamine resins. Since melamine resins do not react as
quickly to the addition of acid, the pot life of the paint
can be prolonged by decreasing carbamide resins and
increasing melamine resins.
Due to the influence of heat or an acid catalyst,
amino resins dry by reacting to another binder and to
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1.1.3 Epoxy resins
In epoxy resins, polyamides and polyamines are
generally used as hardeners. Polyamides provide better
water resistance than polyamines, but dry more slowly.
A binder can be dissolved in water or a solvent,
or can be solvent-free. As their UV resistance is
relatively poor, epoxy paints are generally used as
primers, for example under polyurethane paints.
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1.1.4 Urethane resins
(isocyanate-hardened)
1.1.8 Water-borne binders, i.e. dispersions
Urethane resins dry aided by a chemical reaction.
Polyurethane paints can be either of the onecomponent or the two-component type. They can also
be water-borne.
As one-component lacquers dry, the isocyanate they
contain reacts with air humidity.
Such paints are generally not manufactured, since
the paint’s pigments contain some moisture that reacts
with the binder when the paint is stored. For the same
reason, gloss-reducing flatting agents cannot be added
to lacquers.
In two-component paints, the binder can be either
polyester-type or acrylate. Acrylate-based paints dry
faster than those that are polyester-based. They also
require less hardener, but their chemical resistance is
lower.
Aliphatic isocyanates lead to slow drying, but the
resulting paint yellows to a lesser degree. Aromatic
isocyanates are clearly better suited to indoor paints.
1.1.5 Nitrocellulose
Nitrocellulose, i.e. cellulose nitrate is made of cotton or
wood. Nitrocellulose paints dry physically as solvents
evaporate from the paint film. As a binder, nitrocellulose
is both hard and fragile. Therefore, flexible binders or
plastizers must be added to nitrocellulose paints for
elasticity. Nitrocellulose can also be added to acid
catalysed paints to speed up the drying process.
1.1.6 Cellulose acetobutyrate (CAB)
Cellulose
acetobutyrate does not yellow like
nitrocellulose. This makes it suitable for lacquers that
are applied on top of paints, since this does not alter the
shade of the paint. CAB lacquers applied to wood retain
the wood’s pale color, but are not suitable for application
on dark wood species or dark stains.
1.1.7 UV-hardened acrylate resins
In manufacturing UV-hardened acrylate resins, the
acrylic acid is allowed to react with another resin.
This is how epoxy acrylates, polyester acrylates and
polyurethane acrylates are made, for example. These
binders harden in a few seconds in the UV oven as a
result of UV radiation. Such binders are also used in
water-borne products of the dispersion type.
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Acrylates form the most important binder type,
polystyrene acrylates being a cost-effective alternative.
Polyurethane dispersions are also used to improve
chemical and abrasion resistance. Due to their high price,
however, their use is limited. The binder is dispersed in
water as small particles. In physical terms, latex paint
dries when its water evaporates and the particles stick
together, forming a cohesive film. Dispersion paints
continue to gain in importance in terms of solvent
emission reduction.
1.2 Pigments and fillers
Pigments provide tone and coverage. Pigments and
fillers can be manufactured in different ways.
Although materials found in nature, such as red iron
oxide, calcite, talcum and kaolin, can be pulverized,
most pigments are manufactured synthetically.
Pigments protect paint film and wood from the
sun’s UV radiation. They are traditionally divided into
inorganic and organic pigments. For example, inorganic
pigments include iron oxides, carbon black and titanium
dioxide. Titanium dioxide is the most commonly used
pigment in the paint industry. Organic pigments include
phthalocyanine blue and green pigments, for example.
Inorganic pigments generally provide good
coverage, although their coloring capacity is not as
good as that of organic pigments. Organic pigments are
brighter in color; in addition, their resistance properties
vary greatly.
Pigments are used for making tinting colorants used
for tinting various base paints. Different base paints either
contain a precisely defined quantity of white titaniumoxide pigment, or none at all. Tinting colorants and the
tinting system produce an almost limitless quantity of
shades from a few base paints.
Fillers do not provide coverage, since their refractive
index is too close to the refractive index of the paint’s
binders. When dry, fillers are normally white, but when
fully wetted with binders they become transparent.
They are used to adjust the paint’s viscosity, and the
hardness, gloss, filling capacity, and mechanical and
physical properties of the paint film. Among the most
commonly used fillers are calcite, kaolin and talcum.
Synthetic amorphous silica is the most commonly used
flatting agent.
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1.3 Thinners
1.4 Additives
Except for water, thinners are Volatile Organic
Compounds (VOC). They may consist of individual
solvents or solvent mixtures. Thinners reduce the paint’s
viscosity to the desired level.
Thinners are used to control the paint’s application
properties, leveling and drying. Thinners evaporate
from the painted surface as the paint dries. The
solvents contained in the paint determine the paint’s fire
classification. Furthermore, the volatility characteristics
of solvents determine the paint’s application properties,
film formation, leveling and drying.
The dissolving power of solvents varies in such a
way that different solvents dissolve different binders.
Both the product data sheet and label will indicate the
most suitable thinner for the paint. Indeed, the wrong
thinner can ruin the paint, for example due to the binder
not maintaining its soluble form, but solidifying in the
paint instead. Therefore, the solvent that is the slowest
to evaporate in the paint must always be that which is
suitable for the paint’s binder mixture – otherwise the
paint’s gloss will be significantly reduced.
In water-borne paints as well, small quantities of
slowly evaporating solvents are generally used. In such
a case, they are called co-solvents. These soften the
binder particles and make them stick together to form
a cohesive film during the drying phase. The harder the
binder, the more co-solvents are needed in the paint in
order for the paint to form a cohesive film without cracks
and border grooves. The co-solvents used slow down
the drying of the paint.
Additives are used to improve the different properties
of paint. Paint products generally contain very small
quantities of additives, although they can critically
improve the various properties of the paint. Additives
may also be necessary for facilitating the manufacture
of paint.
In alkyd paints, driers are used to speed the drying
of the paint film. Thickeners are used in water-borne
products. Thickeners provide the paint with the desired
viscosity and prevent it from sagging when applied.
Anti-mould agents are added to outdoor products. In
addition, dispersion agents may be used to facilitate the
grinding of pigments into the paint; levelling agents may
be used to provide a smoother painted surface without
fish eyes. Wetting agents are used to ascertain that
the paint is capable of wetting the substrate in order to
produce a cohesive and even paint film. Adding antifoaming agents will ensure that no air bubbles remain in
the paint film. Photo initiators are added to UV products
in order to allow them to dry in the UV light.
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3
2. Different types of coatings and their
application methods
P
aint products can be divided into different groups
based on their intended purpose of use. In the wood
industry, putties, stains, wood stains, wood perservatives,
lacquers, paints, hardeners and thinners are commonly
used.
are normally not used alone – instead, one or two layers
of lacquer are applied on top.
Different shades of stains are available based on several
different color cards. Both solvent-borne and waterborne stains are available on the market. The following
should be borne in mind when staining:
• Check whether a current color model is available in
order to ascertain whether the resulting shade is correct.
Stain color models fade, yellow, or change color due to
the passing of time and because of light exposure.
• Wood species and different timber lots affect the final
colour of the stain. Check to ensure that the stain used
accomplishes the right shade of colour for the wood
material used.
• Sanding affects the final shade and appearance of
the stain on the wood. A coarser sanding will open the
wood’s cell tissue more, enhancing the absorption of the
stain into the wood and colouring it more evenly. Sanding
too finely may close the wood surface, causing some of
the stain to remain wet on top of the wood. This will lead
to a blotchy appearance, especially in large items.
• The application method affects the strength of the
stain.
• The chosen lacquer type affects the final shade of the
stain.
• Stains can be diluted with a solvent, causing the color
to become a lighter shade.
• Stains can also be mixed with lacquer, giving the
stained surface a more even colour. However, the stain
will no longer highlight the differences between spring
and summer growth rings. Color differences appearing
on unevenly coloured wood can be evened out in the
same way.
• A wetting lacquer is the primary choice for use on top
of stain.
• The sanding of the sealer is facilitated by the use
of stains that cause the substrate to swell as little as
possible.
2.1 Putties
Products that are very thick in terms of their viscosity are
commonly called putties. They are used for filling
the holes, crevices and cracks in raw wood, and for
trimming the wood surface. The putty must be easy to
apply, filling capacity, and as non-shrinking as possible.
Putties can be applied either with a putty knife or
capacity using a putty box and either a light or heavy
putty machine.
Figure 1. Putty knives used for applying putty.
Alkyd resins, nitro cellulose, latex and epoxy and
urethane resins can be used as binders for putty. Putties
are available in either colorless or white form, or in
various brown colors resembling those of different wood
species.
2.2 Stains
Stains are soluble dyestuffs or pigments or solvents that
contain pigments. They are used to stain wood surfaces
without covering the structure and pattern of the wood.
Due to the low surface tension of stains, a deep and
even staining is achieved in most types of wood. Stains
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2.3 Wood preservatives
Wood preservatives are finishing materials, whose
purpose is to protect a wood surface in outdoor use
from mould, decay, fungal organisms and bluing. Wood
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stains ordinarily contain a binder and organic biocides.
A wood preservative should penetrate deep into the
wood and prevent it from absorbing excessive moisture.
The deeper the active ingredients are made to penetrate,
the better they will protect the wood material. In order for
the wood preservative to be absorbed as deeply into
the wood as possible, the product’s viscosity must be
low. A vacuum device can also be used to enhance
absorption, the low pressure pushing the wood stain into
the wood. Wood stains may be used in either colourless
or colourful form. Tinted wood preservatives contain
weatherproof coloured pigments.
also reduce the yellowing that results from the impact of
UV light.
2.4.2 Top lacquers
The most important properties of top lacquers are
mechanical abrasion resistance and chemical resistance. Lacquers suitable for outdoor use must also be
weather resistant and able to withstand the dimensional
changes of the wood, which means they must be elastic.
A similar elasticity is required of lacquers intended for
humid conditions.
2.5 Paints
2.4 Lacquers
Lacquers are finishing materials that form a transparent
film as they dry. The wood industry commonly uses acidcatalysed, polyurethane, water-borne and UV-hardening
lacquers.
The substrate and the structure and form of the item
to be coated affect the choice of lacquer and finishing
method. Furthermore, the compatibility of the lacquer
and wood type must also be taken into consideration,
since incompatibility may cause colour disturbances,
among other things. For example, acid catalysed lacquer
will turn green on palisander. Pre-treatment, drying and
gluing of the substrate also impact on the end result of
lacquering.
Using thin lacquers, i.e. those with low solids content,
produces a result – as the lacquer penetrates into the
pores of the wood – that adapts to the form of the wood
surface. In open-grained wood, thin lacquer should
always be used in order to avoid the formation of airbubbles. When lacquers with a high solid content, and/
or thick layers of lacquer are used, the wood’s surface
will easily adopt a “dead” appearance.
Ordinarily, the same raw materials are used in paints as
in lacquers. The main difference between the two is that
in paints, coloured pigments are used, while in lacquers
they are not. The pigments provide the shade and
coverage characteristic of paints. Shades of colour are
created in a controlled manner using a tinting machine,
tinting formulas, different coloured pigments and base
paints. In addition, paints have a better filling capacity;
they also protect the substrate more effectively against
e.g. UV light.
Often, it is mistakenly thought that paint will cover
up all defects discovered in the substrate during pretreatment. In painting, preparatory work and choosing
the right method and paint is a prerequisite for a highquality end result.
There are sealers and top paints, just as there are
priming and top lacquers. The most important properties
of primers are easy sandability, quick drying and good
filling capacity. Correspondingly, the most important
properties of top paints are mechanical abrasion
resistance and chemical resistance.
2.6 Hardeners
2.4.1 Sanding sealers
The most important properties of sanding sealers are
easy sanding and quick drying. The wood industry
commonly uses one-component sealers that are easy to
use and quick to dry.
Due to wetting, the wood substrates colors will turn
more or less dark depending on the type of lacquer used.
This wetting causes the substrate to darken. A stronger
wetting of the wood may be desirable when lacquering
stained surfaces or dark wood species. When lacquering
light coloured wood species, less wetting is desirable,
allowing the wood substrate to remain naturally light.
The degree of lightness can be increased by mixing a
white titanium-dioxide pigment with the lacquer – this will
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Hardeners are substances that are mixed into the
paint, producing a hardening reaction. Hardeners can
be divided into hardeners and catalysts. A hardener
participates in the drying reaction, forming a film with
either the lacquer or the paint. On the other hand, a
catalyst only catalyzes, i.e. speeds up the reaction – it
does not participate in the reaction itself.
In polyurethane and epoxy coatings, a hardener is
used as a second component, while in acid catalysed
coatings a catalyst is used. In dosing both the hardener
and the catalyst, detailed instructions should be closely
adhered to in order to obtain a successful result. The
amount of hardener required is always calculated
individually for each product.
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2.7 Thinners
Thinners are used primarily for achieving a form for the
coating which allows it to be applied on the desired
substrate. They are also used to dissolve the binder and
give the coating good leveling properties. Thinners can
also be used to affect the wetting of the substrate and to
facilitate the formation of a flawless film by adjusting the
speed of evaporation. As a result of quick evaporation,
the film settles quickly. Slower evaporation allows the
film to level off more evenly, but also introduces a greater
risk of sagging.
In an industrial line, painted products containing
slowly evaporating solvents also become more difficult
to stack. The thinner is often chosen bearing in mind the
customer line speed and the stacking speed.
Thinner is usually a mixture of fast, medium and slow
solvents. The dissolving power of the solvent may be
limited to one or more types of binder.
Thinners are formulated in order to ensure that each
coating receives the intended properties. For different
paints and lacquers, only the thinner intended for a
given paint or lacquer should be used. Most finishing
problems occur as a result of unsuitable thinners or their
being used in the wrong quantity. When using thinners,
the recommendations mentioned in the product data
sheet should be followed. Issues related to the protection
of workers and the environment also affect the choice of
thinner.
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3. Wood structure and different
types of wood
3.1 Wood
In finishing, different wood species have different
properties. For the best possible result, one should be
aware of the special properties of various wood species
before finishing. The properties of different wood species
also determine for which use each species is best suited.
Birch, for example, is relatively hard and elastic, which
is why it is often used as veneer in plywood and bent
furniture structures. On the other hand, its resistance
to humidity and decay are low, and it is not therefore
recommended for outdoor use as such.
Wood species suitable for outdoor use include spruce,
larch and pine. They withstand changes in humidity
without cracking. Also, their wood material is resinous,
which protects them against external strain. However,
unfinished wood material is never recommended for
use outdoors. Coniferous wood may, however, be used
for indoor products, such as panels, while pine may be
used in furniture.
3.1.1 The structure of wood
Wood’s
structure
consists of the core,
core wood, heartwood,
sapwood and bark.
The
core,
which
consists primarily of
starch, stores nutrients
for the next phase of
annual growth and
branches. In the lower
trunk, the core is dead.
Core wood, which
consists of the 10-15
innermost annual rings,
Figure 2. The structure of wood
has a different cellular
structure to later rings. This wood always contains knots
and cracks easily. In terms of its quality, it is the weakest
part of the trunk. Heartwood is dead wood material that
acts as a supporting “pillar” for the living sapwood. In
many wood species, heartwood is darker than sapwood,
and is usually the most valuable part of the trunk. As
I N D U S T R I A L
the tree grows sturdier, the share of heartwood grows.
Sapwood, i.e. the laburnum, is living wood, where water
and nutrients are transferred from the roots to the top. In
light wood species, such as spruce, birch and aspen,
heartwood and sapwood are barely distinguishable from
one another. The bark consists of several layers, and is
the outermost layer of the trunk.
Springwood, summerwood, annual rings, pith rays,
and the phloem and cambium located under the bark
layer also form distinct parts of the structure of wood.
Springwood is formed early in the growth season, as the
tree begins to create new annual growth. Springwood
can be distinctly seen as a lighter ring in the wood
material. In hardwood, the width of springwood is almost
constant regardless of the speed of growth of the tree.
Summerwood is formed in the wood material during
the summer. It is darker in color than springwood, due
to the tree growing faster during the spring, and having
a looser cell structure compared to the summer growth
season. In softwood, the width of summerwood is almost
constant. Moreover, dense and hard wood is desirable
in terms of quality. Good-quality hardwood is the result
of fast growth, while slow growth is advantageous for
softwood. Together, springwood and summerwood are
known as the annual rings. Summerwood is harder than
springwood, and therefore the quantity of summerwood
in the wood material has a significant impact on the
wood’s resistance properties. Thicker cell walls and
smaller lumens also contribute to the better resistance
properties of summerwood, making it denser than
springwood.
In the cross-sectional area of many wood species,
thin lines can be seen running radially from the core
toward the perimeter. These are known as pith rays – their
task is to transfer water and nutrients from the surface
layer to the living, interior parts of the trunk. The size of
pith rays varies according to the wood species. In many
wood species, pith rays are invisible to the naked eye,
whereas in others, such as oak, they are clearly visible.
When the wood is split in line with the ray, a mirror image
of the pith rays can be seen in the splitting surface. This
kind of decorative patterning is typical of oak and maple,
among others.
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7
3.1.2 Branches
Normally, branches develop at the root of the top
growth; hence, they originate in the core. These kinds
of branches are known as core branches. In coniferous
trees, these branches grow in regular layers known as
curly branches. The growth of branches in spruces is
not quite as regular – instead, smaller sprigs grow in
between layers of branches. These are often called
pin branches in sawn timber. The core branches of
deciduous trees do not develop as regularly – instead,
they branch out without a set order.
In terms of finishing, the branches of hardwood
trees have no significance. However, the branches
of coniferous trees present special requirements for
finishing. Coniferous wood felled in the spring, early in the
growing season, is likely to continue transferring the tree’s
nutrients for a long time after the tree has been felled. In
a coniferous tree, this kind of resinous material is apt to
seep out rather easily at the knots. Resin as such does
not cause quality defects in finishing, but the extractives
released from the knot color the finishing material yellow.
This is particularly
visible when the
goal of finishing
is to lighten the
wood material or
paint it white. The
shade of the knot
changes to yellow
after only a few
months of aging.
This yellowing of
the knot cannot
be
completely
prevented;
it
is
possible,
however, to slow
down the dyeing
by choosing the
right
finishing Figure 3. Pine branches.
materials.
3.2 Properties of wood
into consideration when designing structures and details.
Springwood is more porous than summerwood, which
means it shrinks more drastically when drying. As a result
of intense drying, the wood material splits crosswise.
With finishing, the wood’s reactivity to humidity can be
limited and the useful life of wood products thereby
extended.
3.2.2 Humidity
The moisture content of wood has a significant impact on
the wood’s strength below the saturation point. Generally
speaking, the strength properties of wood improve when
the moisture content diminishes. The moisture content
of wood intended for indoor use should be 8-12%, while
that of wood intended for outdoor use should be 12-15%.
Before the wood is processed and finished, its moisture
content should be within the aforementioned limits in
order to ensure the high quality and longevity of the final
product.
Storing and using wood in humid circumstances for
long periods of time exposes it to damage caused by
mould, fungi and decomposition. In practice, this can be
prevented using a well-functioning finishing compound
and well-designed structures.
3.2.3 Temperature
The mechanical properties of wood diminish upon
heating. When exposed to high temperatures for long
periods, the strength properties of wood diminish.
Nowadays, this is used to advantage in treating wood
thermally. The strength properties of thermally treated
wood are not as good as those of untreated wood –
although at the same time, the humidity resistance,
dimensional stability and decay and fungal resistance
properties improve.
3.2.4 Time
Aging does not diminish wood’s capacity for resistance.
With age, however, micro-organisms that live in wood,
as well as a continuous load, diminish its resistance
properties. Constant changes in circumstances and
strain diminish resistance more than stable circumstances
and strain. Therefore, it is important – especially when
external stress is present – to protect the wood surface
by using finishing materials.
3.2.1 The living wood
Wood is an anisotropic substance, i.e. it swells
and shrinks in different directions in different ways.
Longitudinally, its reactivity to humidity is low, although
its transverse reactivity may be high. This must be taken
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3.3 Wood species
The most common wood species used in the wood
industry include pine, spruce, birch, oak, beech and
ash. In addition, several other wood species are used
for various purposes, based on properties such as
appearance, lightness and resistance capacity. In
addition to the aforementioned wood species, aspen,
elm, juniper, linden, rowan, goat willow, alder, bird
cherry and maple are used in the wood industry. Some
wood species cannot be used due to lack of sufficient
quantities of wood material in good condition.
3.3.1 Pine
Pine has spread to nearly all of Europe, and vast areas
of Siberia. Dark heartwood is relatively decay-resistant,
while sapwood is lighter. On the
other hand, pinewood is quite
hard – to a large extent due to its
rapid growth speed. Pinewood
is therefore not very elastic and
breaks easily.
Wood withstands fluctuations
in humidity relatively well without
cracking. Pinewood that is
suitable for the carpentry industry
comes from a tree that has a
breast-height diameter of at least
25 cm, and where the portions
that are knotless, dry-knotted
and healthy-knotted are clearly
visible. Air-dry timber weighs
approximately 540 kg/m3. Highquality pinewood is evenly
Figure 4. Pine.
colored.
Pinewood is used for interior decoration, carpentry
products, boat building and as a raw material in the log,
construction and packaging industries. It is also used
for manufacturing furniture, panels, windows and doors,
and as a raw material for decorative and utility items.
Additionally, it is used for making moldings, cakes of
glue boards, panels and wood boards.
From the standpoint of the
carpentry industry, the disadvantages
of spruce include its knottiness and
resinousness. High-quality spruce
wood is used as raw material for
acoustic instruments. Air-dry timber
weighs approximately 450 kg/m3.
Grade-A spruce wood is of an
even color; the surface should be
smooth and lump-free, and the
thickness of the annual growth rings
should not exceed 3 mm. This kind of
flawless wood is used to manufacture
furniture, doors and windows, as well
as panels and moldings.
Figure 5. Spruce
3.3.3 Birch
Downy birch is straighter-grained than silver birch, and
therefore easier to split. Birch wood is light in color and
the grain pattern is not very visible.
In this case, heartwood cannot be distinguished
from sapwood based on color. The wood is hard and
elastic. Moreover, birchwood requires thorough drying,
as it decays easily. Quick drying also causes the color
to darken. Birchwood does not withstand humidity well,
but it is homogenous and easy to process, shave and
polish. Its finishing, dressing and dyeing properties are
excellent as well.
The structure of the cell tissue
of flamy birch is otherwise normal,
except for the direction of the grains
in the wood – it alternates in a
wave-like manner in alignment with
the ray and tangent. Air-dry timber
weighs approximately 600 kg/m3.
Due to its excellent processing
properties, birch wood is much in
demand as a raw material in the
furniture industry, as well as in
carpentry and turning shops. Birch
is also the primary raw material for
the plywood industry and other
sheet industries.
3.3.2 Spruce
Figure 6. Birch.
Spruce wood is light and its yellowish heartwood is not
clearly distinguishable from the sapwood. Moreover,
spruce is straight-grained and less elastic than birch.
The wood shrinks a little as it dries, but twists and warps
more than pine. Spruce splits easily, but is quite resistant
to changes in humidity.
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9
3.3.4 Oak
Oak is common across Europe and parts of Asia.
Sapwood and heartwood are clearly distinguishable
from one another. Sapwood is thin and whitish. The
colour of heartwood varies from a white-yellowish colour
to medium brown; generally, it is reddish and glossy and
darkens at a later stage. Pith rays are clearly visible in a
cross-sectional cut.
The wood material is strong, hard, quite heavy and
densely grained. Due to its high tannic-acid content,
insects do not usually damage it. Finishing is easy with
different products, although some water-borne products
may have the undesirable effect of turning the colour of
oak greenish. This is due to raw materials in the lacquer
that are unsuitable for oak. Oak is lacquered both as
an open and as a closed (filled) lacquer surface. In an
open lacquered surface, the tubes of oak remain visible,
while in a closed surface an effort is made to fill all
tubes, making the surface completely even. Heartwood
is extremely resistant to decay and highly waterproof.
The wood must be dried
carefully in order to
prevent it from cracking.
When dry, processing is
easy, although cracking
may occur even after
drying. Air-dry timber
weighs
approximately
750 kg/m3.
Oak wood is an
excellent material for
making furnishings, and
is often used to make
kitchen furniture, doors
and windows. ParquetFigure 7. Oak
floor manufacturers are
the primary users of oak wood. Oak makes a fine raw
material for parquet floors, as it is sufficiently hard.
It is also used to make veneers and furniture, and as
wooden raw material for boats. On account of being
decay-resistant, oak continues to be used as a building
material for sail boats.
3.3.5 Ash
Ash grows in moist locations in Europe, as well as in
Caucasia and Asia Minor. Ash wood is tough, heavy and
hard. It withstands bending well, hauling exceedingly
well, and is extremely shock-resistant; its cut-resistance,
however, is poor. Moreover, its heartwood and sapwood
are difficult to distinguish from one another. Sapwood is
thick and almost white. The wood is unevenly patterned
10
T I K K U R I L A
by various shades of color,
making it highly decorative.
Ash wood dries quickly,
and therefore, cracking or
breakages are unlikely to
occur. In artificial drying, it
is best to use moderate heat
to ensure that the wood is
not dried too fast. Ash is not
weatherproof. Ash wood
is difficult to saturate and
should therefore not be used
outdoors. The bulk density of
air-dry timber is approximately
690 kg/m3.
Ash is easy to finish. Like
Figure 8. Ash
oak, it can be lacquered
with either an open or closed lacquer surface. Ash
wood remains rather light, even when lacquered. In the
carpentry industry, ash is mainly used to make parquet
floors and doors for kitchen cabinets. Additionally, it can
be used to make furniture, veneers, sporting goods and
weapons.
3.3.6 Other wood species
Other wood species are used in small quantities in the
carpentry industry. Because of its hardness and light
coloring, maple wood is used mostly for parquets, and
to some extent for furniture. Larch is used primarily for
outdoor structures, such as patio furniture, terraces,
bridges and jetties. Among the best qualities of larch is
its high resistance to humidity and decay, which is why it
is used to replace pressure impregnated wood.
Aspen has been traditionally used to build benches
for saunas, as its wood material does not conduct heat
or chip. Aspen is also used to make furnishings, sporting
goods and toys, as well as decorative and utility items.
Elm is used to make furnishings, parquets and stairs. Elm
wood is hard, heavy, tough and highly decay-resistant.
It is also highly decorative with elaborate patterning.
Linden is easy to process and is used to make furniture,
as well as artistic and decorative items. The wood is
straight-grained, light and relatively soft and crackresistant. Due to its lightness, it is used for the structural
parts of furnishings that are hidden from view.
Alder is an excellent material for the carpenter and is
used to make furniture and utility items. It is also especially
well-suited for making sauna benches and wall paneling.
Nowadays, rowan is utilized in the carpentry industry to
make furniture and utility articles. As a carpenter’s wood,
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I N D U S T R Y
the value of rowan is on the rise, due to the wood’s relative
hardness, resilience and elasticity. Bird cherry is used to
some extent in furniture making. This wood is relatively
soft, elastic and easy to process, giving it the properties
required of a wood suited to bendable structures. The
basic color of curly birch is a light yellowish tone, with
inconsistent patterning. Its annual rings are wavy and
blotched with brown curly cells. The wood is hard and
difficult to process. It is used for high-quality interior
decorating and furniture; to some extent it is also used
to make parquet floors.
In addition to these domestic wood species, many
other wood species are used in Finland, as well as in
other countries – some of them grow wild in Europe,
and some in Asia and Africa. Some of the best-known
foreign wood species include mahogany, teak, merbau,
acacia, walnut, abachi, doussie and wenge. When
finishing untypical wood species, it is recommended
that an adhesion test be performed in advance, since
these wood species may contain wood tanning materials
that reduce adhesive capacity. This also represents an
opportunity to check the dyeing capacity of the wood
before finishing larger surfaces.
are heat-treated in order to increase their stength.
3.4.2 MDF and HDF boards
MDF and HDF boards are frequently used in objects
where different patterns are created on the board material
surface. The kitchen furniture industry uses MDF boards
(Medium Density Fibreboard) in painted doors. To some
extent, MDF boards are also used in furniture parts with
a level surface, such as table tops when it is desirable
to rework the edges of the pieces to make them more
prominent. HDF boards (High Density Fibreboard) are
used in objects that require a higher degree of humidity
resistance than that of MDF, for example, in exterior
doors.
MDF board is manufactured in the same way as
fibreboard (3.4.1). The main difference is the multiplicity
of the wood used in MDF boards. Different wood species
and their mixtures, as well as poor-quality wood, twigs,
thin waste wood, hardwood and any wood material not
used by the wood industry can be used in MDF boards.
The properties of MDF boards, then, are the sum total of
all the materials used to manufacture them.
3.4.3 Plywood
3.4 Board materials
Various board materials are used in the wood industry as
wall materials, in door and window constructions and as
a framework for furniture and furnishings.
3.4.1 Fibreboards
The raw material for fibreboard comes primarily
from wood industry waste wood. Paper wood from
birch is also used to some extent. Fibreboards are
divided into three groups according to their densities:
hardboards in excess of 800 kg/m3, semi-hard boards of
300-800 kg/m3 and soft boards under 350 kg/m3.
Fibreboards are intended for inside lining, while
soft boards are also used as thermal insulation and
windshield panels.
In Finland, fibreboard is manufactured using what is
known as the wet process. Wood chips are fiberized and
the resulting pulp pulverized. The binding between the
fibers is based primarily on couching. Water is added to
the fibers; sometimes alongside additives such as glue.
The wet pulp is directed into a wire, where excessive
water is squeezed out of it. After the fibrous web has
passed through the back-up cylinders, it is cut into
sheets that are either dried (soft boards) or pressed
(hard and semi-hard boards). After pressing, the sheets
I N D U S T R I A L
The plywood industry produces birch and conifer
plywood and different types of mixed plywood, whose
surface veneer is made of birch. The construction
industry uses plywood for moulds, floors, ceilings and
walls. It is also used in the conveying-equipment industry
and for making furniture. When finishing the products, a
coating developed for each specific purpose is used.
The plywood industry also manufactures finished
components for direct delivery to the end user.
The first phase in plywood manufacturing consists
of bathing the logs in warm water. Next, the logs are
peeled and cut into shorter pieces for the turning
machine. Both ends of the short log are attached to the
turning machine. The blade, which is the same width as
the log, cuts the log into a continuous, cross-directional
“veneer mat”. The mat is then sliced into sheets, dried
and sorted into different grade classifications. Next, the
mat is mended and extended as needed, and finally,
Figure 9. The structure of plywood.
W O O D
F I N I S H I N G
11
glued into boards. The boards are then pressed, dried
and sawn to measure. The surfaces are also sanded
and often refined with different coatings.
3.4.4 Chipboards
Chipboard is made of wood chips and sawdust, and is
used for many different purposes: in construction, it is
used for structures, interior decorating boards, lining,
and fixed furnishings; in the construction-carpentry
industry, chipboard is used for furniture and closets. The
furniture industry uses chipboard as a framework for flat
parts of furniture.
Chipboard manufacturing begins with drying the
wood chips and sorting them according to size. Then,
the chips are joined together with glue that hardens
when heated. The chip and glue mixture is strewn to
form layers of pre-form sheets, which are hot-pressed
into a continuous sheet. Next, the sheets are cooled
and the edges sawn. Finally, the sheets are cut into
saleable size, sanded and sorted. When manufactured,
the sheets contain either three or multiple layers – the
chips that form the surface layer are generally thinner
and smaller than those in the middle layer.
OSB (Oriented Strand Board) is a large chipboard, in
which the wood strands are oriented longitudinally. This
board was developed in North America, and European
production is growing rapidly. OSB is commonly used in
some European countries, even for lining outside walls.
In terms of its purposes of use, OSB competes with
conifer plywood.
12
T I K K U R I L A
Figure 10. The structure of chipboard made of wood chips
and sawdust.
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4. Special requirements of different base
materials in finishing work
T
his section addresses the special requirements and
properties of the wood materials most commonly
used in finishing work in the wood industry.
furnishings, the difference is that paints used for HDF
are typically outdoor-proof. Otherwise, painting an HDF
board does not differ from painting an MDF board.
4.1 MDF board
4.3 Plywood
Nowadays, the furniture industry frequently uses
MDF board for kitchen and other furniture. In kitchen
furnishings, MFD boards are used to make doors, and in
furniture, multiform pieces of sheets.
In kitchen furniture, most door models are grooved,
i.e. the dense MDF surface material is cut open with the
CNC machine blade. The resulting coarser MDF sheet
material presents the toughest challenge in terms of
finishing. When painted, the cut MDF board moistens,
raising the grains upright. This is highly visible when
using water-borne paints.
The result is that the bottom of the cut groove is one of
the most challenging objects for finishing work. The best
quality is achieved by ensuring that the cutting blades
and equipment are in the best possible condition when
cutting the board. Problems that occur while cutting the
board cannot be corrected at the finishing stage.
In practice, the best result in painting a groove is
achieved by using solvent-borne paints and by sanding
the bottom of the groove after each finishing treatment.
Customarily, a combination of two primers and one toppaint is used for grooves.
In water-borne systems, the best result is achieved by
spraying a very thin first layer and drying it quickly. The
first layer is not sanded; instead, a second, thick layer
is painted on top of it using a primer, which is sanded
before painting the top coat. These measures aim to
prevent grain-raising from occurring twice. A waterborne product will raise the grain upon each application.
In terms of furniture parts, the same rules apply as in
doors of kitchen furniture made of MDF.
Plywood is made primarily of rotary-cut veneer by gluing
veneers on top of one another, with the grains crosswise.
Rotary-cut veneer presents unique requirements for
finishing plywood. It is full of grain-aligned cracks
containing a lot of air. These cracks are clearly visible
when staining the plywood with certain stains.
The air in the cracks creates air bubbles in the
lacquered or painted surface. This may constitute a
significant visual defect, especially in boards intended
for interior decorating purposes. In technical plywood,
small air bubbles do not affect other properties of the
paint surface. These air bubbles can be eliminated by
pre-heating the board, changing to a slower thinner or
slowing down the drying process.
Crack defects that emerge during staining are
difficult to correct. Indeed, these defects emerge only
after staining – the flawed appearance is caused by the
stain being absorbed into the edges of the crack in a
different manner to even surfaces. Generally, the crack
remains lighter than the rest of the surface. An effort
can be made to correct staining errors by adding stain
in the sealer and top lacquer, enabling the masking of
the contrast between the lightness of the crack and
the rest of the surface. Another correction method is to
add lacquer to the stain before beginning the staining,
causing the stain’s more even absorption into different
parts of the surface veneer.
In other respects, the finishing of plywood is similar
to that of veneered boards.
4.2 HDF board
HDF board is used especially in manufacturing exterior
doors. The material is denser than the MDF board,
making it easier to paint. Also, grain-raising is not as
pronounced. When compared to paints used for kitchen
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13
4.4 Solid wood
4.4.1 Pine and spruce
Pine and spruce are nowadays commonly used as a lining
material for both the exteriors and interiors of buildings.
Additionally, furniture made of pinewood continues to be
popular, especially in Central Europe. When finishing the
surface of pine and spruce, the focus of attention should
be on the drying temperature. The surface temperature
of resinous wood species in the drying oven should not
exceed +35°C, as the resin in the wood will then retreat
from the wood’s surface, causing a visual defect on the
paint and lacquer surface. Momentarily, pine and spruce
may withstand temperatures as high as +50°C without
any problems arising in their resin.
Pine is also frequently used for making window
parts. In such parts, particularly when using water-borne
paints, one of the most frequently occurring problems
is the yellowing of the knots caused by the resinous
material. This yellowing happens over time and can
never be completely prevented. Different types of paint
prevent the appearance of the yellowing in different
ways. This defect, however, is aesthetic only, and the
paint film is highly resistant to external stress, even after
the yellowing has begun.
4.4.2 Oak and ash
Should glazing colours, however, be the only
alternative, it is best to test the desired shade on a wood
sample first. If the wood sample appears blotchy, the
intermediate sanding of the birch must be made either
coarser or finer. If the surface is too coarse, stain will be
absorbed unevenly; if the surface is too fine, stain will
remain in puddles on top of it. Differences in color can
be evened out by adding glazing colours color in the
surface lacquer.
4.4.4 Tropical hardwood
Unusual tropical wood species have become more
common in the parquet industry and in terrace
furnishings. The finishing of these wood species can
be performed as with normally used finishing materials.
However, they are most frequently oiled in order to retain
and even deepen their characteristic colour tone.
However, in some tropical wood species the high oil
content makes it difficult for the lacquer to adhere to the
wood surface. In such instances, the only way to determine
adhesive capacity is to conduct a test lacquering using
different products. The paint manufacturer can also
perform its own product development in terms of special
adhesive sealers suitable for particular wood species.
In oak and ash, the grain structure is open and relatively
deep. Air is easily retained in the pores of these wood
species; in the drying zone, it leaves the wood, breaking
the lacquered surface and creating air bubbles. Airbubble creation is customarily prevented by pre-heating
and using slower thinners.
Oakwood contains tannic acid; water-borne lacquer
may dye it green. In such an instance, the wood material
does not withstand certain ingredients in the lacquer. In
order to prevent such dyeing, a different product must
be used for base sealing, or another type of lacquer
used. The dyed lacquer layer must be sanded off totally
and lacquering begun again from the start.
4.4.3 Birch
As a light wood species of even quality, birch is relatively
easy to finish. Indeed, growing use of transparent
shades of color has made achieving an even surface
more difficult. Since the difference in color in springwood
and summerwood in birch is so slight, glazing colours –
especially in dark shades – is not recommended. Birch
will look better when stained conventionally.
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T I K K U R I L A
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5. Sanding
S
uccess in wood sanding depends on many factors,
such as the wood species, wood quality, the
roughness and velocity of the sanding belt, the backing
paper, and the direction of sanding. Success in sanding
is always measured by the total sum of the optimal
circumstances achieved in all of the factors involved.
For example, increasing the speed of the sanding belt
does not necessarily improve sanding quality, since at
a high belt velocity, the grit overheats and may break,
causing the wood surface to burn.
5.1 Sanding methods
In making wooden products, several different sanding
methods are needed. These may include calibration
sanding, wood sanding, intermediate sanding or exhaust
sanding. In industrial finishing, all of these methods may
be required. The surface quality is primarily affected by
wood sanding and intermediate sanding, whose purpose
it is to prepare the wood surface for finishing, and to even
out the sealer or primer layer for applying the top coat.
Calibration sanding is used to balance the differences
in the thickness of sheet-like pieces. Exhaust sanding is
primarily used prior to repainting, or in removing a failed
finish. Sanding can also be separated into manual and
mechanical sanding. The same basic principles apply
in each case, although manual sanding is used more
frequently for multiform pieces, such as chairs; in such
instances sanding belts with a cloth-woven backing are
also used more often.
5.2 Sanding belts
The sanding belt consists of a backing material,
grains and a binder. The properties of the different parts
can be altered and combined to achieve a different
sanding result.
Paper is largely used as back material on account
of its cost-effectiveness. The thickness of paper may
vary. There are six different grades of paper from A to
F – the difference is in the weight in grams per m2, and
therefore also in thickness. Grade A paper is under
90 g/m2 and Grade F paper is over 250 g/m2. Another
I N D U S T R I A L
alternative support material is backing material made of
cloth-woven thread; it is stronger and more stable. Cloth
material is recommended for objects which require a
very strong and elastic backing material. Cloth materials
can be divided into elastic, soft, strong, extremely strong
and polyester cloths.
Both natural and synthetic grains and abrasives are
available. Synthetic grains are harder and more durable.
They are therefore used more frequently, especially
in sanding machines. Grains are graded based on a
hardness scale from 1 to 10, where the hardest material
– diamond – has a value of 10. The most commonly used
grains include aluminum oxide and silicon carbide, and
to some extent zirconium oxide. Harder grains may be
necessary when sanding harder wood materials, such
as oak, ash, maple and merbau.
Figure 11.
Aluminium-oxide-based grains
yield a plough-like sanding
result.
Silicon-carbide based grains
yield a shaving-like cutting
result.
In sand papers, the roughness of the belt is
indicated through numerical values. In wood sanding
and intermediate sanding, degrees of roughness vary
between P16-P1000, where the smaller numbers indicate
a coarser sanding, and the larger numbers indicate finer
sanding. Calibration and exhaust sanding are normally
performed at a roughness of between P36-P60, as
an effort is made to remove wood or paint material as
effectively is possible.
The sanding roughness generally used in wood
sanding is between P80-P220. In sanding, it is customary
to move in the direction of finer sanding one step at a
time. If calibration sanding is performed at a roughness
W O O D
F I N I S H I N G
15
of P60, the next sanding phase can be done at a
roughness of P80 or P100, i.e. when moving forward,
one stage of roughness can be skipped. Should the
shift be made from P60 directly to P120, the scratches
caused by the coarser sanding may remain visible in the
finalized surface.
When moving onto the final stage in wood sanding
before finishing, a finer degree of coarseness is used.
Oak and pine are normally finalized on the relatively
coarse P150 belt, while birch, beech, ash and other
wood species are normally finalized at a belt coarseness
of P180 and even P220.
In intermediate sanding, the roughness normally
used is between P220-P1000. The finer the wood
sanding before finishing, the finer the roughness that
can be used in intermediate sanding. When beginning
intermediate sanding, it is best to adhere to the rule of
not skipping more than one degree of roughness at a
maximum.
Closed surface structure.
Semi-open surface structure.
Open surface structure.
Figure 12. Different surface structures.
The abrasive is glued to the back material with a binder,
such as animal-based glue. However, in industrial
production plastic glue is most commonly used
nowadays, as it is more resistant to the heat created
during sanding. The surface structure of sanding
products alternates between a closed and open surface
structure. In a closed surface structure, the grains are
close to one another; in a semi-open surface structure
they are a little further apart; and in an open structure
they are quite far apart. In coniferous wood species,
an open surface structure is often used, preventing the
resinous wood material from blocking the sanding-belt
structure as quickly as when a closed surface structure
is used.
In intermediate sanding, stearate-coated belts can be
used, when the desired result is a surface of extremely
16
T I K K U R I L A
high quality. Sanding with stearate-coated belts is less
aggressive in the beginning and the result is more even
throughout the service-life of the belt. Also, the belt
surface is dust and dirt repellent, making it less likely to
become clogged. As a result, the belt is more durable,
thereby reducing the total costs of sanding. However,
the use of stearate belts in old sanding machines is not
necessarily recommended. The reason for this is that in
old machines the belt is likely to become overheated,
as the melting point of stearate is between 110-140°C,
depending on the material used. In order to function at
optimal capacity, the belt must be efficiently cooled.
Using antistatic belts reduces dust problems. With
cleaner machines, sanding belts and surfaces, finishing
will yield better results. A reduced dust load enhances
the working environment, as well as safety, since the risk
of a dust explosion is reduced.
5.3 Instructions for sanding
••The item to be sanded must be of an even thickness
throughout in surface sanding.
••Do not use excessive power when sanding. With too
much power, the sharp grits will break and slow down
the sanding. Light sanding will also produce a betterlooking surface, reducing the consumption of lacquer or
paint. This saves electricity as well.
••To save on sand paper, use an elastic sanding pad
or roller. An appropriate elasticity can be achieved by
using thick rubber of 40-50 Shore hardness. This makes
surface sanding easier and prolongs the service life of
the sanding belt. In calibration sanding, a significantly
harder rubber material is used in order to achieve
sufficient processing capacity.
••Pre-sand the wood so that it is completely free of glue,
dirt and possible scratches. Especially when waterborne stains are used, glue remains will not always dye
as well as when using solvent-borne stains.
••The sanding must be appropriately divided between
pre-sanding and fine-sanding. If, for example, a
system with three sanding belts is used, the first belt
can be used to remove 60% of the stock removal
produced by the total sanding, while the second
and third belts are used to remove 30% and 10%,
respectively. In a two-belt system, the division can be
75% with the first sanding belt, and 25% with the second
belt. If separate sanding machines are used, the objects
can be turned around between sanding phases, and the
sanding can be done in the opposite direction.
••Fine-sand only until the traces of pre-sanding have
disappeared – no more. This will result in the best
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I N D U S T R Y
Figure 13. Strain caused by sanding on three sanding belts.
possible surface quality.
••The more finely the wood is sanded, the better the
quality of the surface will be. In some cases if the wood
is sanded too finely, it may turn a solid wood surface
shiny, preventing stain from being absorbed evenly into
the wood and creating a spotty result.
••Fine sanding can also be used to optimize the use of
finishing materials. Prior to finishing, sanding dust must
be removed from all surfaces, as it can cause roughness
on the finished surface.
••Use as strong a backing material as possible, as
stronger material is more durable.
••Check the running direction of the sanding belt; this is
indicated with arrows on the back of the belt.
••Loosen the sanding belt when the machine is not in
use. This will prevent the belt from tearing and make it
last longer.
••Use the right belt speed in mechanical sanding:
20-30 m/s for plywood sanding, 18-22 m/s for hardwood
and MDF/HDF, 12-18 m/s for softwood (pine), and
5-15 m/s for coating. In sanding coatings, lower sanding
speeds are used primarily when sanding water-borne
and other thermoplastic coatings. Hard lacquers and
paints – for example UV drying products – can be
sanded using the higher values given.
••Check to ensure that the sanding dust suction is
working well in order to maintain the cleanliness of the
machine and the object to be worked on. Oak wood dust
is classified as a carcinogen, which means that dust
removal must be working well at all times. The dust of
other wood species is also being investigated, and may
also be declared a carcinogen.
••In order to achieve the best possible surface and
evenness, it is important that a sharp sanding material be
I N D U S T R I A L
used. By optimizing its use, the sharpness of the sanding
material can be ensured for the maximum length of time.
This will make the sanding highly cost-effective.
••The air humidity of the sanding space should be
over 35%, including during winter. The ideal level is
approximately 40-45%. An appropriate and even air
humidity level is highly significant, also for the wood.
••Making the right choice (roller, pad, hardness, quality of
belt, speed) helps to achieve the best possible service
life for sanding belts. The best solution is found after
several test runs.
••Do not open the belt packages until they are to be
used.
••Build horizontal storage bars for the unpacked belts.
••Do not allow sanding belt surfaces to rub against each
other.
••The appropriate storage temperature is 15-25°C; the
air humidity should be 35-60%.
••Do not store the belts near a source of heat or against a
cold exterior wall, where steam is likely to condensate.
••Do not store the belts in direct sunlight.
Figure 14. The operating principle of the wide-belt roller
sanding machine.
Figure 15. The operating principle of the wide-belt pad
sanding machine.
W O O D
F I N I S H I N G
17
6. Finishing and coating methods and
equipment
6
T
he most important criteria for choosing finishing
equipment are dictated by the requirements of
the products manufactured and the factory’s overall
finishing needs. This issue will be further addressed in
the section entitled Finishing lines. In order to build a
well-functioning finishing line, one must also be familiar
with the properties of the individual application devices.
Each device has its pros and cons that affect its ability
to function in the line. Finishing materials may also give
additional requirements.
One of the most critical attributes of any device is
its operating efficiency. Operating efficiency in painting
refers to that portion of the paint to be transferred onto
the surface to be painted. However, operating efficiency
is always a theoretical numerical value that varies a
great deal, depending, for example, on the shape of the
product to be painted, line velocity, air circulation and
the person who performs the painting work.
5
7
4
3
3
1
2
1.Pump
1. Pumppu
2.Ejector
2, Ejektori
3, Poistoilma
3.Exhaust
air
4, Vesikalvo
5.Overflow edge
5. Ylivuotoreuna
6.Conveyor
6. Riippukuljetin
7. Vedenerotuslamellit
7.Lamella water separators
4.Water curtain
Table 1. Operating efficiency of different painting methods
and overspray.
Figure 16. The operating principle of the wet filtration booth.
Operating
efficiency %
Loss
%
6.1 Spraying and spraying equipment
Powder coating
97 – 99
1–3
6.1.1 Spraying booths
Roll
97 – 99
1–3
Roller coating
97 – 99
1–3
Brush
95 – 97
3–5
Curtain coating
95 – 97
3–5
Dousing
95 – 97
3–5
Vacuum
95 – 97
3–5
Dipping
90 – 95
5 – 10
Bell
85 – 95
5 – 15
Hot air conventional spraying statics, 135 kV
85 – 90
10 – 15
Cold air-assisted Airless statics, 75 kV
70 – 80
20 – 30
Hot air conventional spraying statics, 75 kV
70 – 80
20 – 30
Cold air conventional spraying statics, 75 kV
60 – 70
30 – 40
Hot Airless
55 – 70
30 – 45
Cold Airless
50 – 60
40 – 50
Hot air conventional spraying
45 – 55
45 – 55
Cold air conventional spraying
30 – 40
60 – 70
Painting method
18
T I K K U R I L A
When the objective is to achieve as excellent spraying
result as possible, spraying must be performed in a
spraying booth. The primary purpose of a spraying
booth is to remove the over-sprayed paint particles and
solvents from the work space in order to make the work
environment more comfortable. The spraying booth must
also remove lacquer and paint particles from outgoing air
and thereby protect the environment against impurities.
Both dry and wet filtration booths are available. In a
wet filtration booth, exhaust air is conducted into water,
into which chemicals have been added. As a result,
paint particles either rise to the surface or sink to the
bottom. The choice of chemical type depends on the
type of paint and products used.
In a dry filtration booth, exhaust air goes through
a dry filter that consists of sheets or disposable filters
made, for example, from paper or fiber glass and placed
in a labyrinth-like pattern. The separation capacity of dry
filters is 70-90%. Since the pressure difference is small,
at approximately 10 kPa, the air flow is relatively low
(0.5 m/s using a regular sprayer and 0.3 m/s using an
electrostatic sprayer). A dry filter is used primarily in small-
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I N D U S T R Y
scale production, although dry booths have become
more commonplace, even in larger production facilities,
due to their being more economical to operate.
Normally, in a large production facility, the majority
of finishing work takes place in a line dedicated to the
purpose. A spraying booth is used, for example, in
edging the products, or to apply special colors and
paint small quantities. Depending on the paint products
used, the separated sediment that settles in the bottom
or rises to the surface of the wet filtratine booths may be
hazardous waste. Therefore, it cannot be deposited in a
waste disposal site, and must be brought to a hazardous
waste disposal plant to be destroyed. This constitutes
a separate cost item for the company. A laboratory
specializing in such investigations can determine
whether the sediment qualifies for deposit at a waste
disposal site.
6.1.2 Conventional spraying
Conventional spraying is based on dispersing paint
by using compressed air. Compressed air must be
as free from condensation and impurities as possible.
Compressed air guns have controls for adjusting the
width of the spray and fine-tuning the quantity of paint.
The advantages of air conventional spraying include:
•• Even thickness of film.
•• Formation of a high-quality surface.
•• Quick adjustment of paint quantity and spray
pattern.
•• May be used with most types of paint.
•• Low purchasing and operating costs.
The disadvantages of conventional spraying include:
•• Low capacity.
•• Requires large quantities of diluents.
•• Large over-spraying.
•• Large formation of paint mist.
•• Substantial loss of paint.
Before starting actual spraying, it must be ascertained
that the has a suitable viscosity. In addition, the
compatibility of the spray gun’s nozzle combination with
the intended paint must be checked. The spray pattern,
known as the fan, is adjusted by test-spraying on a flat
board, for example. At the same time, the amount of
paint and pressure are adjusted to the desired level.
6.1.3 Airless spraying
In airless spraying, the coating is dispersed as mist by
using significant differences in pressure levels. The highpressure coating is pressed through the small opening in
the nozzle (see the tables on pages 19 and 20), whence
it is dispersed as mist, due to the difference in pressure.
The required pressure is generated with a piston pump
or a membrane pump. The spraying pressure is usually
70-140 bars.
Based on their driving power and operation, airless
spraying can be divided into the following categories:
•• Pressure-operated devices based on piston
pumps.
•• Electrical devices based on membrane pumps.
•• Combustion-engine-equipped membrane pumps.
•• Electrical mechanically driven piston pumps.
•• Electrical hydraulic-oil piston pumps.
The advantages of airless spraying include:
•• High capacity.
•• High viscosity may be used.
•• Quantity of paint is minor.
•• A single spraying generates a thicker dry film than
when air conventional spraying is used.
Table 2. Dimensions of the nozzle used in high-pressure
spraying and the piston nozzle diameters when spraying
different types of paints and coatings.
Material to be
sprayed
Nozzle
Spray gun filter
diameter
Ø
Wood stains and other
products of low
viscosity
0,007” (0,180 mm)
0,084 mm
0,011” (0,280 mm)
0,100 mm
0,150 mm
Alkyd paints and acid
catalysed paints
0,013” (0,330 mm)
0,100 mm
0,150 mm
0,150 mm
0,310 mm
0,015” (0,380 mm)
Latexes, epoxies and
other thick-film paints
Fire-retardant paints,
spray fillers,
special coatings
Figure 17. Conventional spraying
0,018” (0,460 mm)
0,310 mm
0,021” (0,530 mm)
0,310 mm
0,023” (0,580 mm)
0,310 mm
0,026” (0,660 mm)
0,310 mm
0,054” (1,370 mm)
0,560 mm
The nozzle spraying angle is chosen according to the shape of the
structure to be painted. The most frequently used spraying angle in
wood-industry painting is 40°-50°.
I N D U S T R I A L
W O O D
F I N I S H I N G
19
Table 3. Effects of changes in airless spraying.
Increase
in pressure
Increase
in density
High
viscosity
Increase in
temperature
Increase in
surface tension
Evenness of spray
Improves
No significance
Capacity
Increases
Decreases
Deteriorates
Improves
No significance
Decreases
Increases
No effect
Enlarges to a
certain point
No significance
Diminishes
Enlarges
Diminishes
Drop size
Diminishes
No significance
Enlarges
Diminishes
Enlarges
Spray velocity
Increases
Decreases
Decreases
Increases
No significance
Spray impact force
Increases
No significance
Decreases
Increases
No significance
Nozzle erosion
Increases
No significance
Decreases
Increases
No significance
Spraying angle
The disadvantages of airless spraying include:
•• Some types of paint sag easily.
•• The spray gun cannot be used to adjust the shape
of the fan; instead, adjustments are performed by
choosing a suitable nozzle. Fine tuning can be done by
adjusting the product’s viscosity and by varying
the application pressure.
•• Purchase price is high compared to conventional
sprays.
6.1.4 Air-assisted airless spraying
6.1.5 Electrostatic centrifugal method
Using this method, the coating is fed into the centre of
a rotating plate or bell that rotates at a speed as high s
50.000 r/min (see Figure 19). As a result of the centrifugal
force involved, the coating is carried to the edge of the
plate, where it is dispersed into particles and charged
with electricity. Compressed air may be used to assist in
directing the coating mist in the desired direction.
The coating mist is drawn on the surface of the
grounded object as a layer equal in film thickness
throughout. The paint loss is minor.
In air-assisted high-pressure spraying, both highpressure and conventional spraying are used to
advantage. This method differs from high-pressure
spraying in, for example, the following ways:
- The pressure ratio of the feeder pump is 1:15
(1:30 in high-pressure spraying)
- The effective pressure is 30-70 bars
(100-140 bars in super-high-pressure spraying)
In air-assisted super-high-pressure spraying, the
high-pressure nozzle is used to create a fan, into
which atomizing air is blown at a pressure of 2-3 bars
while the feeding rate is 35-50 l/min.
Figure 18. Airless spraying.
20
T I K K U R I L A
Figure 19. Electrostatic centrifugal method
6.1.6 Electrostatic spraying
Electrostatic spraying utilizes the electrostatic field that
forms between two objects of different voltage levels
(see Figure 20).
The more powerful the electric field, the larger is the
desired electrostatic impact. The intensity of the field is
dependent on the following factors:
1.The voltage between the spray-gun and the
object.
The object to be coated is grounded and the
difference in voltage is formed by charging the coating
particles. In spray-guns, 60 kV voltages are commonly
used, since at higher voltages the sprayer draws some
of the particles toward himself.
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I N D U S T R Y
2.The spraying distance
The shorter the distance, the stronger is the electric
field. In practice, however, the spray-gun should not
be brought too close to the object. Depending on the
kind of device used, the most appropriate distance is
50-250 mm.
Figure 20. Electrostatic spraying.
Since the electrical conductivity of wood is poor, due
to the low humidity levels (8-12%) used in the furniture
industry, an appropriate grounding is not achieved.
The conductivity of the coating used for electrostatic
spraying of wood must not be as low as the conductivity
used for spraying a metal object – instead, an effort is
made to use the conductivity of the coating itself to direct
the charge to the ground. In order to do so, the coating
must always begin at the grounding point.
Should the electrostatic system fail to function
properly, improvements may be sought by, for example:
•• Increasing moisture level of the wood.
•• Enhancing surface-conductivity of the wood
(priming coat).
•• Increasing the relative humidity of air in the spraying
space.
•• Using several clean grounding points.
•• Using smaller-size hangers.
•• Reducing the particle size of the coating.
•• Reducing the air flow in the spraying space.
The special equipment needed and the difficulty of
spraying the inside corners may be numbered among
the disadvantages of the method.
6.1.7 Spraying with heated coating
In order to make the spraying of the coating as easy as
possible, its viscosity can be reduced. In most cases,
varying quantities of water or solvents are added in order
to achieve the appropriate viscosity. Another method is
to heat the coating (to 40-80°C) before spraying in order
to make it sufficiently fluid. Warming the paint allows
for savings in water or solvent; also, the coating to be
sprayed does not need to be diluted quite as much. The
solid content of the coating is increased; thereby the
filling capacity of the coating is increased, it provides
better coverage, does not sag as easily, and dries
faster.
Conventional spraying, airless spraying and
electrostatic spraying can be done using heated coating.
Heating is based on water, steam, electricity or warm air,
depending on the device used.
6.1.8 High-volume conventional spraying
(HVLP spraying)
This method is based on conventional spraying. The
difference, when compared to normal conventional
spraying, is that the pressure used is lower, which
reduces the back stroke.
High Pressure
Air Spray
Figure 21.Airless spraying.
Both water-borne and solvent-borne
coatings are available for the electrostatic spraying of
wood. Electrostatic spraying may be combined with
other spraying methods (Airless spraying, conventional
spraying, bell spraying etc.). The advantages of this
method include:
•• Minimal overspray.
•• The object to be coated does not need to be sprayed
from all sides (time savings).
I N D U S T R I A L
In HVLP-spray guns, the coating is atomised by using
a large amount of air and low pressure. The air nozzle of
a customary spray gun has numerous, small holes. The
air nozzle of the HVLP spray gun has a few large holes,
and the pistol is equipped with large air channels that
cause smaller resistance to air.
In regular compressed air spray guns, a pressure of
3-5 bars is used to atomise the coating. In HVLP spray
guns, a pressure of only 0.5-0.7 bars and a large amount
of air atomise the coating equally efficiently.
W O O D
F I N I S H I N G
21
6.1.9 Two-component spraying
High Volume-Low
Pressure Spray
Figure 22.High-volume low-pressure spraying.
Figure 23.Airless spraying.
High Pressure
Air Spray
Figure 24.High-volume
low-pressure spraying.
High
Volume-Low
Pressure Spray
This method is commonly used for fast-drying twocomponent coatings. The primary task of the twocomponent device is to dose the paint and hardener
and mix them prior the spray gun. Due to the coating
drying so fast, the part of the equipment where the paint
is mixed with the hardener must be washed immediately
after finishing the job. For this reason, two-component
devices are equipped with a washing pump.
The two-component spraying equipment consists
of a working pump, a dosing pump and a mixing unit
(static, dynamic). Additional equipment available
includes feeding pumps, heaters and washing pumps.
The benefits of this method include:
• Always the right mixing ratio.
• The mixture is always of even quality.
• Only a small amount of solvent is needed.
• The system is a closed one.
• The amount of paint loss is reduced.
• The containers are large.
• Safety at work is improved.
• Colors can be changed at reduced labor costs.
• Washing time is shortened.
6.1.10 Automatic spraying
RUISKUTETTU
MAALI
SPRAYED
RUISKUPAINT
TETTU
BACK
TAKAISIN
STROKE
LYÖNTI
HAJOITUSILMA
Figure 25.Atomising air at a
paineessa
4.1
baria
pressure
of 4.1
bars.
HAJOITUSILMA
paineessa 4.1 baria
RUISKUTETTU
MAALI
22
TAKAISIN
LYÖNTI
Figure 26.Duotech at a
DUOTECH
pressure
of 0.7 bars.
SIIRTOTRANSMISSION
HYÖTYSUHDE
EFFICIENCY
LOSS
TETTU
MAALI
BACK
TAKAISIN
STROKE
LYÖNTI
HUKKA HUKKA
SPRAYED
RUISKUPAINT
HUKKA HUKKA
HUKKA
LOSS LOSS
HUKKA
MAALI
TAKAISIN
LYÖNTI
TRANSMISSION
SIIRTO- SIIRTOHYÖTYSUHDE
HYÖTYSUHDE
EFFICIENCY
LOSS
HUKKA HUKKA
With this method, it is possible to heat the atomising
as well. Since in this case the coating is not heated, it will
last for a long time without gelling. This method has all
the benefits of thermal spraying.
SIIRTOHYÖTYSUHDE
paineessa 0.7 baria
DUOTECH
paineessa 0.7 baria
T I K K U R I L A
Coating can be rationalized in the same way as other
production methods, for example, through automation.
Automatic spraying makes sense when there are long
series of similar objects in need of coating. The shape
and raw material of the object, and the type of paint to
be used, determine the choice of spraying method. The
most important requirement of an automatic equipment
is reliability of service.
Among the advantages of automatic spraying are the
following:
•• Large capacity.
•• Even quality.
•• Reduced labor costs.
•• Painting is highly economical.
•• A better working environment.
The linear, traverse and carousel spraying machines
are among the most commonly used automatic
equipments. In automatic linear spraying, there are
usually three fixed spray guns located above the
line. The speed of the linear spraying line is about
30-50 m/min.
A traverse is a machine that moves spraying guns
in a linear direction at a constant speed. Normally, the
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I N D U S T R Y
6.1.11 Robot spraying
Figure 27. Automatic horizontal traverse spraying machine.
stroke length and moving speed of the traverse can be
adjusted. Adjusting the distance of the spraying guns is
usually done manually. There are two kinds of traverses
– vertical and horizontal – so named based on their
operating direction.
Vertical traverses are used, for example, in the
window industry, while horizontal traverses are used
in the door and furniture industries. Vertical traverses
are usually equipped with one spray gun and there are
several traverses in a row. These are usually connected
to electrostatics spraying equipment. Bell spraying has
become even more common. The line speed varies
between 1-3 m/min.
In a horizontal traverse, there are most often four
spray guns; the line speed varies between 1.5-9 m/min.
A carousel spray machine may have, for example, twelve
spray guns – half of which are used for lacquers and half
for paints. The line speed is the same as in the horizontal
traverse system. Nearly all of the before mentioned
spraying systems can be connected to an automated
spraying machine.
Using a robot is an excellent solution at limited
conditions, and also when the results must be absolutely
high-grade. Painting robots are equipped with a builtin microprocessor and an electric-circuit module. All
of the movements and coating impulses the spray gun
needs are registered in an electronic monitoring unit
in accordance with what is known as a program arm.
The program arm precisely repeats the movements of
a skilled spray painter; alternatively, the movement path
can be programmed using a computer.
In robot spraying, special attention must be paid
to the wear and tear of the spray nozzles. In order to
maintain an even painting quality, the robot must spray
the same amount of paint at all times. This is achieved
with a flow meter. Another, more practical method is
to test the erosion level of the nozzles during use and
replace them when necessary. As a result of wear and
tear, the nozzlehole may be increased to the next, bigger
size after spraying only 500 liters.
Figure 30. The action radius of a robot sprayer.
Vaakaraverssiruiskuautomaatti
Figure
28. Horizontal traverse spraying machine.
I N D U S T R I A L
Figure 29. Robot spraying.
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23
6.2.2 Vacuum coating
6.2 Coating methods
6.2.1 Curtain coating
The curtain coating machine is a very fast painting
equipment, and can be used for work at several different
line speeds. When painting with a typical curtain coating
machine, the mat speed is 70 m/min, while the speed
of the painting line is normally 10-20 m/min. The curtain
coating machine is usually equipped with an accelerator
mat, causing the speed under the curtain film to be
higher than on the line itself. Flat surfaces and slightly
profiled panels and moldings are especially well suited
for curtain coating machine. Due to its large capacity, a
curtain coating machine can be used separately; or it
can be connected to the entire coating-line system.
The amount of coating to be applied can be adjusted
exactly by changing the pump pressure, the distance
between the curtain coating lips and the speed of the
conveyor mat. Since the loss of material is minimal in
comparison to spraying, coating with a curtain coating
machine is an economical alternative. A very even
coating result is achieved by keeping the viscosity of
the coating at the standard level during coating. An
automatic viscosity meter can be attached to the curtain
coating machine to keep the viscosity at such a level. It
is important that the curtain coating machine is not kept
in a draughty location, since this may cause the coating
film to flutter, and – in the worst case scenario – to break
completely.
Coating additives, the purpose of which is to level
the coating film, may cause the surface tension of the
coating to drop too low, causing the film to cut off. In
a traditional curtain coating machine, the application
rate may vary between 70-170 g/m2. The viscosity may
vary from 30 seconds to 60 seconds when measured
with DIN 4 cup. The same is true for a machine known
as an overflow-curtain coating machine, which is more
effective than a traditional curtain coating machine in
terminating any foam and air bubbles the coating may
contain.
Curtain coating machine with roller was developed
primarily for water-borne and UV hardening products.
They are more likely to foam than other products,
especially in curtain coating machines. Roller curtain
coater allow for smaller application rate (40-120 g/m2)
than other types of curtain coating machines. Small
application amounts are difficult to applicate in practice,
since thin films are very sensitive to air draught.
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T I K K U R I L A
The vacuum machine is specifically intended for waterborne and UV hardening coatings. This machine is not
suited for solvent-borne products, due primarily to the
fast evaporation of certain solvents.
Objects, for which a vacuum coating is most suitable,
include moldings, frames and claddings. One of the
advantages of this method is that the coating penetrates
every place of the object being worked on. In vacuum
treatment, the coating is pumped into the coating
chamber. The coating is then atomized using suction
air and suctioned out of the chamber from above and
allowed back into circulation. In this way, an even paint
mist is formed inside the chamber; the item to be coated
is then feeded through the mist. During this process, it
will be coated all-over. The object then exits the coating
chamber through a precisely measured opening, which
wipes the extra coating off the object’s surface. The
suction air flowing into the chamber along the object’s
surface carries the extra coating with it, back into the
chamber and into circulation.
Figure 31. Vacuum-coating device suitable for water-borne and
UV-hardening coatings.
The amount of coating that remains on the surface of the
object depends on the following factors, among others:
•• The amount of coating in the coating chamber
basin.
•• The speed of the air current at the exit opening.
(the greater the speed, the larger the amount of
coating that is drawn back into the chamber).
•• The speed at which the item passes through
(the slower the speed at which the object moves, the
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I N D U S T R Y
more time there will be for the coating to be drawn
back into circulation).
• The surface quality of the object to be processed
(smooth/uneven).
• The quality of the coating used (viscosity, adhesive
capacity etc.).
The popularity of vacuum machine is increasing
significantly; moreover, coatings of an ever more reliable
functioning capacity have been developed for them.
6.2.3 Roller coating application
Roller coater is an alternative to the curtain coater
for even surfaces. This method is quick, simple
and economical. On the other hand, one roller can
be used to apply a maximum of approximately
35-45 g/m2. Normally, a double roller is required in order
to achieve application rates of 30-80 g/m2.
In a roller coater, the paint is pumped in between the
dosing roller and the spreading roller. In such a case,
the rollers are rotating in opposite directions in terms of
one another (direct feeding). Since the circumferential
speed of the spreading rollers is greater than that
of the dosing roller, the coating becomes “polished”
in between the rollers, and forms an even film on the
dosing roller. The thickness of the film can be adjusted
by changing the amount of pressure between the rollers,
or by changing the running speed of the dosing roll.
In a direct run, slowing down the running speed of the
dosing roller reduces the amount paint, while speeding
up the rotating speed increases that amount.
In a reverse run (i.e. indirect feeding), the changes
are the other way around. The viscosity of the coating
also affects the application rate.
In order to even out the structure of the coating surface
– especially in normal direct-rolling – the dosing roller
can also be rotated in the reverse direction. This reduces
the application rate, which is usually approximately
7-15 g/m2.
In a roller coater, a higher-viscosity coating is used
than in other methods. This results in a higher solid
content, which partially compensates for the smaller
amount spread.
Since the solid content of UV hardening products, for
example, is 100%, they produce an excellent filling rate
for surface with only small amounts application in top
lacquering, the amounts applicated can be as small as
5-10 g/m2.
Several different-quality rubber rollers are available
for roller coaters. Some are more sensitive to certain
solvents than others. If the wrong solvent is used for
washing the rollers, they are in danger of swelling as a
result. If only 100% UV-products are used on the rollers,
only a few annual washings is needed.
6.2.4 Dipping
Dipping is a quick and simple coating method. It is
particularly well suited for priming. Items, for example
windows and furniture parts, can be dipped in the
dipping basin either in parts or assembled. During
dipping, single-component products are ordinarily used
– nowadays, they are also mostly water-borne. Doublecomponent acid catalysed coatings are also suitable for
dipping, provided that a catalyzer especially intended for
dipping is used. In dipping, the viscosity of the coating
is usually 15-20 s/DIN4.
Direct run
6.2.5 Drum-coating
Drum-coating is used primarily for finishing small pieces.
The object to be coated is placed in a rotating drum,
into which a sufficient amount of coating is sprayed
or poured. After coating is completed, the objects are
dried sufficiently to enable their handling; such drying is
done by blowing warm air into the drum. Drum-coating
can also be done in a static container. The coating used
must be of the single-component type.
Reverse run
Figure 32. In roller coating machine, the rollers can be rotated
either clockwise or counter-clockwise, depending on the kind of
surface structure desired
I N D U S T R I A L
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F I N I S H I N G
25
7. Finishing lines
A
s the need arises for large-capacity wood finishing,
the shift must be made from painting a single object
to line-painting. Depending on the product, either
horizontal lines for flat and longitudinal objects may
be used; for large pieces, vertical lines may be used.
Horizontal lines are used in the board industry, and for
finishing flat furniture and longitudinal objects, such as
mouldings, panels and frames. Vertical lines are used in
the door, window and chair industries.
In finishing lines, all of the aforementioned finishing
methods, from dipping basins to vacuums, and from
automatic sprayers to roller coating machines, may
be used. Designing a finishing line is always based
on the shape of the objects to be coated, the required
capacity, the finishing materials used, as well as other
circumstances particular to the company.
The decision on whether to design a horizontal or
vertical line is usually based on the shape of the object
to be treated. In kitchen furniture production, the objects
are usually flat and grooved. Finishing these types of
pieces is normally best done with an automatic sprayer.
A curtain coating machine could be an option, but in
most cases the depth of the groove and the edge profile
are too multi-shaped to be handled with a curtain coating
machine. This would require that the edges be handled
in large volumes before proceeding to the finishing line
– which is unlikely to be economically feasible. Using
an automatic sprayer allows for the treatment not only
of flat surfaces, but of edges as well – in this way, the
bottom of the groove is sure to be thoroughly lacquered
or painted.
Doors too are flat objects. Some doors have a profile,
while others are known as flush doors. However, the door
industry uses both vertical and horizontal lines. A vertical
line has the advantage of large capacity and needs only
a small space at the drying stage. Depending on the
painting system used and the doors’ intended purpose of
use, each line type has its advantages. In manufacturing
interior flush doors, roller coating machine and UVdrying materials can be used, allowing for a relatively
short, flat object line while maintaining its high capacity.
In roller coating machine lines, finishing the edges
must be done as a separate work phase, or the paint
or lacquer surface must be replaced by a edging strip.
The drying ovens usually consist of flash off, drying and
cooling units. In this section, the equipment assembly –
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T I K K U R I L A
the drying oven – includes all three parts, although this
is not always mentioned separately.
7.1 Window industry
In the window industry, objects may be finished as
individual parts or as assembled pre-form frames. The
advantage of finishing the individual parts is that all wood
surfaces receive all finishing layers. When assembled,
the ends of the corner joints are left without finishing –
this may diminish the overall high quality of the finishing
work.
A system based on applying three treatments is
recommended for windows: one layer of wood stain
and two layers of paint or tinted lacquer. The wood stain
may be applied on a separate line by dipping; after
that the wood stain is dried at room temperature or in a
warm oven. This is followed by placing the parts onto a
vertical line, where the first layer of paint is applied with
what is known as the flow-coating machine. After this,
the objects are given a round in the oven, and during
the second round the final surface layer is sprayed with
vertical automatic spraying machine onto both sides of
the objects. An electrostatic device may also be used for
spraying, in which case both sides can be painted from
the same spraying direction.
Depending on the capacity need, the aforementioned
line can also be built as a continuous line, where the
flow-coating device is followed by a separate oven; the
rest of the line is the same as before. If pine is used as
the raw material for windows, the maximum allowable
temperature of the ovens is approximately 45 °C in order
to keep the resin in the pine from causing problems.
Different variations of the same system may be used,
such as:
•• Dip + dip + automatic spraying machine.
•• Flow coating + flow coating + automatic spraying
machine.
•• Flow coating + automatic spraying machine +
sanding + automatic spraying machine.
•• Automatic spraying machine + automatic spraying
machine + sanding + automatic spraying machine.
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Any drain drops must hit a spot where they cause no
aesthetic or operational disadvantage.
7.2 Door industry
Figure 33. A factory line.
Different products are available for different application
methods. When dipping and flow-coating methods are
used, the viscosity of the products is usually relatively
low, and additives are used to prevent foaming. When
spraying methods are used, the viscosity of the products
is higher; therefore the products are thixotropic, which
makes it easier to apply thicker layers without sagging.
Depending on the properties of the products, and when
thicker layers are applied, the objects must be sanded
between applications; as a result, the adhesion is
improved and the surface will be as even as possible.
When parts are painted, the frames used in Finnish
window models can also be painted on a flat object line.
In such a case, linear spraying lines are in common use.
It is easier to operate than the traverse sprayer – the
sprayers are static and turned in a way that allows a
sufficient amount of paint to be applied to all sides of
the object. Following the sprayer, the next in line is a
lateral shift and drying in a flat or multilayer oven or on a
wheeled rack.
When assembled, windows to be coated can be
painted using the above-mentioned equipment and
methods, except for the linear spraying line. In the ovens,
the objects usually turn 45° or 90°, depending on the line,
in order to enable the use of a smaller drying oven. At
the same time, however, the impact of the air circulation
on the drying process diminishes, as the surfaces of the
objects move further away from the air inlets on the oven
walls. In designing the oven, it is important to ensure
adequate air circulation, especially when water-borne
products are used.
In terms of already assembled window parts, attention
must be paid to ensuring the appropriate hanging
position, as well as to the profile design. The parts must
be in a skewed position in order for the extra wood
stain or paint to freely drain into the collection basins.
I N D U S T R I A L
Industrial painting of doors is performed when the
doors are ready and assembled on either vertical or
horizontal lines; the door frames are usually painted on
linear spraying lines. Vertical lines are used primarily for
extensively profiled and heavy exterior doors. Profiled
inner doors with a lighter construction and flush doors
are ordinarily painted and lacquered on horizontal lines.
In profiled doors, primed door skins have become
quite common; these are hot-pressed and glued to
the door frame. Skins are normally painted with waterborne acrylate paint before arriving at the door factory.
Depending on the painting quality, roller coating with
water-borne paint can be used as the first paint layer
on profiled doors. After this, one layer of acid catalysed
paint can be applied with a curtain coating machine or
automatic spraying machine. When painting brown door
skin, at least two layers of paint are usually needed, with
an intermediate sanding in between. After the pieces
have been painted, they go into the drying oven, where
the temperature can be as high as 80-90°C. The door
frame is made of pine wood, which temporarily tolerates
relatively high temperatures.
For flush doors, a similar painting system can be
used, although the most efficient way of painting them
is to use the roller coaters machine line and UV-drying
paints. The line then consists of roller coating followed by
UV lamps to dry the 100% UV product in a few seconds.
The UV paint can be dried completely and after drying an
intermediate sanding must be performed. Alternatively,
the UV paint can be gelled using only half of the energy
capacity, allowing for running the next layer of paint
without sanding.
The following line is an example of a well-functioning
solution. The devices in the line are listed in the running
order:
•• Sanding machine for finishing the wood surface.
•• Light-putty roller.
•• UV oven (gallium, gelling).
•• Roller coater.
•• UV oven (gallium, gelling).
•• Roller coater.
•• UV oven (gallium and mercury, drying).
•• Sanding machine for intermediate sanding.
•• Roller coater.
•• UV oven (gallium gelling).
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F I N I S H I N G
27
The number of UV lamps in the UV oven (gallium and
mercury, lamps) is determined by the desired line
speed. Each lamp must generate a sufficient amount of
energy to gel or dry the previous application. Frequently,
the aforementioned lines are built in a U shape, where
the intermediate sanding is preceded by a lateral shift,
and there is a turning device at the end of the line. In this
way, the doors are completed in two runs.
In place of UV products, water-borne paints may also
be used. In such an instance, UV lamps must be replaced
with a short oven, in order to dry the painted surface.
This will generally increase the length of the line, which
will then need more space as well – otherwise, the line
speed must be reduced. In roller coater lines, however,
UV paints are recommended as top paints, since they
have better mechanical resistance.
Finishing doors on a vertical line does not differ
much from the finishing performed on window lines.
Depending on the chosen finishing system, the painting
can be performed in two sprayings, using what is known
as the wet-on-wet system. In this way, the painting of
both sides of the doors can be completed in one run.
However, drying the thick layer of paint will take longer
time in this case.
Finishing the door frames can be done with similar
devices to those used for the moldings and panels;
please see the item entitled Moulding industry (on page
29) for a description of the finishing lines used for this
work.
and spreading rollers, has a metallic roller for pressing
the spread putty as deeply into the crevices of the wood
as possible. Next, the UV-putty is dried with regular UVlamps, followed by sanding the over spreaded putty off
the parquet panels.
After the putty line, the objects go to the lacquering
line via intermediate storage. This line may consist
of roller coaters, UV-ovens, sanding machines and
IR ovens. The first roller is generally used to apply
UV lacquer, designed for improving the adhesion to
the wood. The lacquer may be 100% or a water-borne
UV-drying product. If water-borne UV lacquer is used, it
must be ensured that the water in the lacquer is allowed
to evaporate before to the UV-drying. In some instances,
this is done using the IR-oven located after the roller
coating machine, or a longer intermediate conveyor.
Hard tropical wood species in particular, such as Iroko,
require a separate adhesion sealer lacquer of their
own.
The rest of the rollers are used to increase the total
amount of the lacquer by approximately 65-100 g/m2. An
entire line may consist of the following devices: sanding
7.3 Parquet industry
Parquet finishing is done primarily with UV curing
materials. To some extent, various waxes and oils
are used, although their share is relatively low. Only
transparent substances are used for finishing parquets,
which means that only mercury lamps are used for UVdrying.
Parquet panels are normally run through two
separate lines. The first line is a putty line, where a thick,
high-viscosity, UV hardening putty is used. The parquet
panels are puttied in order to utilize the raw wood to the
maximum extent possible. Putty can be used to fill the
largest knot holes, scratches and other uneven spots in
the wood. The putty is usually tinted to match the color
tone of the wood as closely as possible.
Calibration sanding precedes the puttying machine
in the line – it is used to make the parquet panels as near
as possible in the same thickness. This is followed by a
heavy putty roller, which in addition to the usual dosing
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T I K K U R I L A
Figure 34. Kährs
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machine for calibration, roller coater, IR oven, UV oven,
light putty machine, UV oven, roller coater, UV oven,
sanding machine for intermediate sanding, roller coater,
UV oven, roller coater, roller coater, UV oven.
In the case of running UV-oils and -waxes, similar
machines are used as in lacquering. Depending on the
ingredients contained in these UV products, more IR
ovens may be needed for evaporation.
7.4 Moulding industry
The moulding industry handles longitudinal products,
which entails new requirements in terms of line
design. Since the speed of the objects at the painting
point is normally extremely high, even as high as
30-40 m/min, the drying of the objects cannot take place
in the same direction as painting. Therefore, subsequent
to the linear spraying line, there is a lateral shift in the
line, and drying takes place either in a multilayer oven,
flat oven or a wheeled-rack oven.
Stains, lacquers and paints are used to finish
mouldings. Different products do not require different
equipment – all products can be applied using the linear
spraying line. Depending on the line speed, the linear
spraying lines are equipped with 3-6 static spraying
guns, aimed so that the objects passing through receive
a sufficient layer of paint on their visible surfaces. The
mouldings are intended for indoor use, i.e. it is not
necessary to apply a separate primer to the back-side
of the mouldings.
Spraying is followed on the line by a lateral shift into the
oven. In addition to traditional convectional drying, these
ovens use IR-drying and UV-drying in the newest lines.
These lines may consist of an linear spraying line, drying
oven and sanding machines. The sanding machines are
usually placed in the front section of the line, before
the linear spraying line, allowing them to be used for
evening out the wood or MDF surface before lacquering
or painting. If only one automatic spraying machine
is available for use, the objects can be completed by
running the moldings through the line twice. With further
development, the line can be constructed in a way that
allows the moldings to be completed in just one run.
In this instance, a minimum of two spraying points and
two ovens are needed. Some moulding manufacturers
even have as many as three linear spraying lines in a
line. The UV oven’s arrival on the lines represents the
newest technology in the moulding industry. This allows
for coating compounds containing less solvents to be
used in lacquering or painting. For sealer lacquering
I N D U S T R I A L
and painting, 100% sprayable UV products are used.
Their advantage is a very high capacity and a short
line, as no evaporation is required and the surface is
ready to be sanded after it comes out of the oven. The
disadvantage of 100% sprayable products is that they
may cause skin irritation, should the continuous care of
the spraying machine`s filters be neglected, or when the
spraying device is serviced and cleaned.
A safer option for spraying is the use of waterborne UV-products, which do not cause skin irritation.
However, these add to the length of the line, as the
water must be evaporated from the wet surface before
drying in the UV oven. On the other hand, an old line can
easily be converted to suit a water-borne UV product
by placing a UV oven after the oven in the line. This
enables the replacement of solvent-borne products with
more environment-friendly water-borne UV products
at relatively low cost, while also enhancing product
quality.
7.5 Finishing sawn timber
Finishing long pieces of sawn timber intended for
outdoor use presents new challenges. Since such
wood is intended for outdoor use, it must be treated
with wood stain, as well as being painted with several
layers of paint. The best quality is achieved when the
priming is done on each side of the wood. This means
that the most durable priming in finishing sawn timber is
achieved with a vacuum machine. The vacuum causes
the wood stain to enter each side of the wood, and the
negative pressure in the vacuum forces it deep inside
W O O D
Figure 35. Tikkurila archives.
F I N I S H I N G
29
the wood, providing the wood with excellent protection
against moulds and fungi.
The vacuum in the line is followed by a lateral shift
into the oven. Since the wooden material being treated
is usually resinous coniferous wood, the maximum
allowable temperature of the oven is approximately
45°C. Following the oven, the next layer of a finely-sawn
surface can be painted without intermediate sanding.
Painting is performed using similar linear spraying lines
to those used in the molding industry. After painting, the
timber goes into the oven via a lateral shift. The ovens
are either of the multilayer type or flat ovens. Sawn timber
can also be treated with automatic spraying machines
only.
7.6 Kitchen furniture industry
With respect to kitchen furniture, finishing work is
nowadays performed on doors and kitchen worktops.
Frame materials are normally made of chipboard that
has a melamine surface. In bathrooms and other damp
spaces, moisture-resistant laminated chipboard is used.
Medium Density Fibreboard is the primary material for
painted doors, while solid wood and veneer is used for
stained and lacquered doors.
Line operations are largely based on the same
principles as lines on which horizontal doors are
processed. As more color shades become available,
staining and special colors are finished in separate lines.
This type of line may consist of a Fladder-type sanding
machine, an automatic spraying machine and a drying
oven. The drying temperature may be a relatively high
60-80°C, since the materials generally used withstand
heat very well. When pine doors are manufactured, the
temperature must be lowered.
Lacquering and painting lines function on the same
principle as in staining, for example. First, a Fladdertype sanding is performed, followed by an automatic
spraying machine and a drying oven, which can be
either a multilayer or a flat oven. Of the two, a flat oven is
used more frequently these days than a multilayer layer
oven. In place of Fladder-sanding, different types of
brush sanding machines may also be used.
In lacquering lines, water-borne UV lacquers are
used instead of solvent-borne lacquers. Therefore, the
drying oven’s principal purpose in a lacquering line is
water evaporation, and the final drying is performed
using UV lamps. When water-borne UV lacquers are
used exclusively, there is no need for a separate cooling
section, since UV lacquered products may be stacked
while still warm.
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T I K K U R I L A
7.7 Furniture industry
Assembly line solutions in the furniture industry depend
largely on the company’s product selection. A company
which manufactures chairs exclusively may invest in an
electro static line with conveyer, while one that offers
comprehensive interior decorating services must finish
multi-form pieces – in the latter instance, one line may
not be enough.
Most chair manufacturers solve the problem by
having a spraying system combined with a traverse line
and a drying oven; the spraying system is most often
an electro static system. Electrostatics enables a higher
effeciency, since in spraying chairs, the amount of
overspray – especially in the case of already assembled
chairs – increases too much when traditional methods
are used. Some manufacturers combine electrostatics
with the use of a robot, allowing for further savings in
materials costs.
If water-borne products are used, traditional spraying
methods combined with different recycling systems can
also be used. These include the cold wall, lack-in-lack
or cylinder-wall systems. The purpose of all of these
is to allow for collecting the over-sprayed material and
filtering and reusing it in spraying. This increases the
material’s utilization ratio as high as 90%.
Flat pieces are normally finished in roller coater lines
using 100% UV-products. In furniture, edge finishing
is normally done in stacks before the line-run, using a
manual spray in the spraying booth. If there are many
different edge profiles – making processing in stacks
difficult – a regular automatic spraying line may be used.
In this way, the edges can be treated while the lacquer
or paint is being applied to the flat surface.
There are many different parts to a piece of furniture –
finishing these may demand special procedures. Some
special parts may exist in such quantities as to make it
more sensible, as well as more economical, to design
particular finishing equipment exclusively for them,
using the dipping method, for example.
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7.8 Other types of wood industry
The choice of equipment for other types of wood
industry is largely based on the demands posed by the
objects being produced. Each solution is then unique
and company-specific, and may not lend itself to being
applied to another type of industrial enterprise. All
application methods may be used, including dipping, a
rinsing machine, curtain coater, spraying systems, roller
coaters, vacuum etc. From the company’s point of view,
it is extremely important to assess the construction and
operating costs of the assembly line and compare them
to the expected operating life of the line.
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F I N I S H I N G
31
8. Drying and hardening
I
n order for the coating to be applied on the substrate,
it must be in liquid form. In principle, drying and
hardening means the transformation of fluid coatings
into solid ones by way of the evaporation of the solvents
and/or the binders reacting chemically.
8.5 Chemical drying
Most coatings used for industrial coating of wood may be
dried or hardened at room temperature. The drying time
is affected by changes in temperature, air circulation
and air humidity.
Based on a chemical reaction, alkyd-amino-resin-based
binder compounds and two-component polyurethanes
dry and harden to form an insoluble film as a result of the
effect of an acid catalyst
or isocyanate hardener. The evaporation of the solvents
and the reaction accelerates at elevated temperatures.
Synthetic alkyds and oils dry and harden as the solvents
are eliminated and the binder reacts (oxidizes) with the
oxygen in the air. The drying speed can be accelerated
to only a limited extent by increasing the temperature.
8.2 Heat drying
8.6 Heat transfer
Drying at elevated temperatures accelerates the drying
process and enhances film formation. In order to
accelerate such drying, the heat and radiation energy
should be increased, thereby accelerating evaporation
and chemical reactions. Heat will quickly increase the
surface temperature of the object, and the drying time of
the coating will be significantly shortened. An increase
of 10 degrees Celsius in the drying temperature will
shorten the drying time by approximately half. Extremely
short drying times are only achieved at temperatures
exceeding 50°C.
Generally, in drying coating, more than one heat transfer
method is used and several may be used simultaneously.
In the drying ovens, both air circulation (= convection)
and IR emitters (= radiation) are often present. In
addition, the air circulating in the oven is often heated
with water-circulation heaters (= convection).
8.1 Air drying
8.3 Drying and hardening
8.6.1 Conduction
Heat is conducted from the warmer object to the colder
one. For example, a pre-heated substrate warms up
the coating film. Solvents evaporate faster and the film
begins to harden from the bottom up.
8.6.2 Convection
A single coating normally contains several binders,
enabling its drying properties to be adjusted to suit
the existing drying line, for example. The solvents the
coating contains also play a role in the drying reaction.
8.4 Physical drying
Air flowing through heated surfaces warms up the
coated object. By enhancing air circulation with blowers
and by directing the air current onto the object with a
steering plate, heat transfer is achieved faster. The
surface temperature rises more quickly and solvents are
evaporated more efficiently.
Certain binders, such as nitrocellulose and celluloseacetobutyrate, dry by evaporating solvents. The film may
be dissolved again using solvents. Drying is accelerated
using elevated temperatures.
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T I K K U R I L A
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I N D U S T R Y
8.6.3 Radiation
In radiation, energy is transferred electromagnetically.
Radiation strikes a surface, is absorbed therein
and transformed into heat. Radiation has different
wavelengths; in drying ovens infra-red radiation (IR),
ultra-violet radiation (UV), electron beam radiation (EB)
and microwave radiation (MOS) are used.
A long-wave IRL emitter is sometimes called a dark
emitter. It is used in conjunction with convection drying,
for example, where heated oven surfaces provide heat
by emitting radiation onto the coated object. Mediumlength IR radiation (IRM) is achieved with an IR emitter;
by changing the temperature, it is often possible to
adjust the radiation power continuously.
Short wave IRK radiation is also used to some
extent. It can be adjusted continuously very fast, and
the radiation power is greater than that achieved with an
IRM emitter.
UV-hardening means hardening a UV-reactive
substance with UV-light, which affects the reactive
ingredients in UV-products, i.e. the photoinitiators which,
as they disintegrate, cause the binder to cross link
(polymerize). Lacquers and colourless UV-products dry
without any problem, using UV-radiation. The pigments
0,38
0,76
Light
Gamma
radiation
X-ray
radiation
1 nm
IRK
IRM
2µ m
1 mm
4µ
Short Middle
IR
IR
UV
radiation
Long-wave IR
IR- radiation
1 mm
1 µm
IRL
Convection
used in paints either absorb or reflect UV-radiation.
White and light colour shades dry well with UV-radiation,
whereas drying some dark, brown and red color shades
is difficult using this method.
In electron beam radiation (EB), the substrate is not
heated; instead, the energy affects only the coating,
which can be a clear lacquer or a pigmented product.
The common denominator for all heat emitters is that
the distance between the heat source and the object has
a major impact on drying. In addition, radiation energy
works only when it comes into contact with the surface.
MOS radiation is generally used to enhance water
evaporation before the actual flash-off zone. It is not
used for solvent-borne products.
IRR
Radio waves
1m
1 km
IRM
IRL
Convection
Lacquer
Wood
Wood
Figure 36. IR radiation is either short-wave (IRK), medium-wave (IRM) or long-wave radiation.
IR-radiation is either short- (IRK), middle- (IRM) or long-wawe (IRL)
I N D U S T R I A L
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33
9. Drying equipment (ovens)
T
he need for drying ovens is dictated primarily
by the plant’s production volume and type. A
variety of drying ovens is available from a simple, lowtemperature wheeled-rack drying room to technically
sophisticated drying ovens. Companies must adhere to
strict requirements in terms of the working environment,
which means that in general the drying oven must be
one that dries the coating at a location other than where
employees are working. Ventilation is always a part of
drying-oven operation – it removes the solvents or water
that has evaporated during the drying process, from the
oven or work space. Environmental regulations, reducing
VOC solvents, and the desire for faster drying times
impact on the product development of drying equipment
and coatings.
3. Drying zone
In the drying zone, a clearly higher air velocity and
higher temperatures are used than in an evaporation
zone. In this zone, the remaining solvents evaporate
from the coating film. In two-component products,
chemical drying begins when the majority of the solvents
have evaporated. It is important that the drying advance
sufficiently far in the oven in order for the coated objects
to be dry for handling on stacking. Should the film
continue to contain too much solvent at the drying stage,
they may cause the surface to boil. IR-radiation is also
used in the drying zone to fasten drying.
4. Cooling zone
In this zone, the object is cooled to a temperature that
allows handling and stacking (under 35°C). It should
be noted that, since in the drying oven heat is stored
inside heavy, massive objects, the surface temperature
should not exceed 35°C, even in stacks. Cooling is an
important part of drying, since in water-borne dispersion,
acid catalysed and polyurethane paints the drying
reaction may not have been completed; in such a case,
stacking may cause shiny spots to appear on the paint
surface. If the temperature inside the stacks is too high,
the painted objects may stick to one another or cause
dent marks in the items at the bottom of the stacks. The
amount of air required for circulation in the cooling zone
is approximately 25,000 m3 per hour.
9.1 Drying tunnels
Regardless of their structure, drying tunnels are
frequently divided into different zones:
1. Pre-heating zone
In the pre-heating zone, objects are heated before
finishing; and are placed in front of the actual finishing
equipment. The intent is, for example, to remove air from
the pores of an open-grained wood substrate, speed
up the evaporation of solvents and level the surface
temperature of the objects to be coated before the
actual finishing.
9.2 Multilayer ovens
2. Evaporation or flash-off zone
In the evaporation zone, the coating is allowed to even
out and penetrate the porous parts of the substrate. Also,
any air bubbles caused by the spraying will burst, and
some of the solvents are removed through evaporation.
In this way, holes caused by air bubbles are prevented
from occurring in the finished surface and the surface
quality is left more even. In the evaporation zone, it is
very important to have a sufficiently large amount of air –
approximately 20,000 m3 per hour. Within the evaporation
zone, the speed and temperature of the air must not
climb too high. Generally, the air velocity is approximately
1-2 m/s and the air temperature approximately 30-35°C
Multilayer ovens are based on convection heat. In
comparison to drying tunnels, their greatest advantage lies
in the relatively small floor area they require considering
their capacity. Multilayer ovens usually have three, four
or six segments. Operations can be divided depending
Figure 37. A layer oven.
34
T I K K U R I L A
O Y
I N D U S T R Y
on the type of product or paint being used. In a threesegment dryer, the first segment is used for evaporation,
the second for drying and the third for cooling. In other
dryer types, the segments for evaporation, drying and
cooling can be divided according to need.
The maximum temperature of a multilayer oven in the
drying zone is usually approximately 80°C. The leadtime varies from 60 minutes to 90 minutes. The drying
time is relatively long compared to the 10-15 minutes
required in drying tunnels. For this reason, the number of
layer dryers in use has continuously diminished.
9.3 IR ovens
In infrared radiation (IR), the wavelength varies between
0.76 µm–1 mm. The wavelengths of IR emitters are towards
the lower end of the radiation range. In IR dryers, either
IRK, IRM, IRL, NIR or OIR radiation is used. Of these,
the first three refer to the traditional division based on
wavelength. L stands for long (4-10 µm), M stands for
medium length (2-4 µm) and K for short (0.76-2 µm)
radiation.
The surface temperature of the IRK-emitter is
approximately 3,200°C, of which 10% is convection
heat and 90% radiation heat. The corresponding values
in an IRM emitter are approximately 800°C, 50% and
50%; and in an IRL emitter approximately 200°C, 90%
and 10%. IRK and IRM emitters require cooling in order
to remain in good condition. With age, the heat emitted
resembles convection heat.
In the most modern ovens, a NIR emitter is placed
in the front section of the oven for quick removal of
the solvents in the paint film. The NIR emitter must be
followed by an evaporation zone. The wavelength of
an NIR emitter is close to the IRL wavelength; it usually
works in cycles to prevent the paint surface in the front
section of the oven from heating up too much. This might
cause a film to form on the surface too early; it might also
cause disturbances in drying.
The specialty of the OIR (Optimized IR) emitter is its
adjustable wavelength. With the help of different emitter
materials and power feeding, an exact wavelength area
particular to each type of paint and substrate material
can be found to ensure the most effective drying for the
type of paint used. In this way, the drying line can be
maximally shortened.
The most commonly used IR emitter is IRM. The
shorter the wavelength of the IR radiation, the more
effectively it penetrates the substrate. For this reason,
I N D U S T R I A L
using IR radiation is not recommended for heat-sensitive
wood species, such as spruce and pine. IRK radiation
may be used, for example, on fibre board or MDF-board.
The radiation emitted by IR emitters always proceeds
in a direct line. Therefore, IR ovens are at their best
when used for drying flat products and the emitters
can be brought to a very close proximity to the object’s
surface.
In comparison to convection ovens, the advantages of
IR ovens include:
•• Shorter drying times.
•• Less energy needed.
•• Less space required.
•• Only the surface is heated, making cooling the
objects easier.
IR-ovens also require evaporation and cooling zones
for smooth operation.
Some putties, water-borne products and stains can
be dried directly with IR. IRM ovens are often used to
preheat the objects as well.
9.4 Gas IR
In some oven types, IR radiation is accomplished by
burning gas and oxygen in a cell-like structure. The
oven is connected to electric preheating to increase the
temperature of the fuel cell matrix sufficiently high – after
that, the flow of gas and oxygen sustains the burning.
The burning reaction is so called flameless burning;
it generates both heat and IRK radiation. The burning
temperature helps in controlling the wavelength of the
IR-radiation that is generated. In this way one can search
for the optimum range for the finishing material used.
Since the cell matrix is hot, it also burns any solvents
evaporating from the surface, thus reducing total solvent
emissions. The IR gas oven can be used with both
solvent-borne and water-borne products.
9.5 MOS-drying
MOS means drying based on microwaves of a certain
wave length in a separate MOS oven. MOS is used with
water-borne products to evaporate water from the wood
surface as fast as possible. The microwaves mobilize
the water molecules in a way that allows it to steam off
the paint surface quickly. The advantage of this method
is an accelerated drying system, where the water wets
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35
the wood or MDF surface for a shorter period, reducing
grain-raising. MOS is used in the evaporation zone,
and a regular drying and cooling zone are needed
afterwards.
9.6 UV ovens
The use of UV products and ovens in wood finishing
has increased significantly in recent years. This is due
not only to environmental factors, but also to the rapid
development of equipment and products. In addition,
UV ovens clearly require less space than other types of
oven.
In a UV oven, products dry and harden in
approximately five seconds. Hardening is based on
the quick reaction of the photoinitiator, binder and any
monomer caused by the UV light. The photoinitiator acts
as the catalyst for the reaction, divided into two reactionsensitive parts in the UV light. These particles, together
with the binder and monomer, begin to form long chains,
of which the lacquer or paint surface is comprised.
400-450 nm wavelength area. Mercury lamps are
also needed for drying paints. Gallium lamps dry
UV-products in greater depth, whereas mercury lamps
give better surface hardness to the paint.
Used UV-lamps can be reused as what are known as
gelling lamps. As a lacquer layer is run on the first roller
of a UV-roller coating line, it ca n be gelled using half of
the amount of energy indicated in the product data sheet.
Using the next roller, another layer of lacquer can be run
on top of the gelled lacquer layer without intermediate
sanding. After the lacquer layer has been dried using
the right amount of energy, it must always be sanded
before running the next layer in order to ascertain the
intercoat adhesion of the layers of lacquers.
UV-lamps require cooling; equipment suppliers
normally take this into consideration in factory
installations. When running too hot, UV-lamps lose some
of their power. UV-drying does not ordinarily require an
evaporation zone. Specialty products, where solvents
have been used to adjust viscosity, as well as waterborne UV-drying products, constitute an exception. A
problematic issue in UV-drying is the drying of different
shades of paint shades. Some dark brown shades
absorb UV-energy, preventing the drying of the paint
surface.
9.7 UVITEC®-curing
Figure 38. UV-drying oven.
Different photoinitiators are used in lacquers and
paints due to the properties of the titanium-oxide in the
paints. Consequently, two different types of UV lamps –
mercury and gallium – are also used in the UV oven.
Mercury lamps are intended primarily for drying clear
lacquers. Their power range is between 80-120 W/cm,
and they function in the 350-360 nm wavelength area.
With age, UV lamps lose some of their output capacity,
resulting in the lamps eventually generating too little
energy to dry UV lacquer. The service life of the lamps
is approximately 2,000 hours, after which they must be
replaced with new ones.
Gallium lamps are used to dry pigmented
products and thicker layers. They function in the
36
T I K K U R I L A
New drying methods for 3D objects have been sought
for a long time. Regular UV-curing has not been effective,
due to problems in extending the UV rays to all of the
surfaces, as well as the great distance of the lamps from
the surface to be dried. UVITEC® drying represents the
newest technology, where low-capacity UV lamps are
used to cure mostly water-borne, UV-curing materials
in carbon dioxide gas. The gas eliminates the negative
effect of the oxygen in the air. Oxygen tends to react with
the binder on the surface of the UV product, making the
polymer chain short and the mechanical properties of
the surface poor.
Carbon dioxide gas has been conducted into a UVdipping basin, into which the objects travel along the
conveyer. The objects are immersed in the gas; thus,
a high level of UV radiation can be achieved in the gas
using lamps and highly reflective mirrors. Hardening
takes around a minute; the output capacity of the lamps
being approximately 20 W/cm.
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I N D U S T R Y
9.8 Electron-beam dryers (EB dryers)
The process known as Electron-Beam Curing (EBC)
involves drying the coating with electron rays. This
process is not widely used in industry. One reason for
the scarcity of EBC equipment lies in high investment
cost. The newest equipment is simpler, requiring an
acceleration voltage of approximately 150 kV; investment
costs still remain relatively high, however. EB radiation is
achieved as a result of the transfer of energy generated
by the accelerated electrons. As the electrons penetrate
the coating, immediate polymerization (hardening) takes
place via a radical mechanism. EB drying does not take
place while oxygen is present and nitrogen is therefore
needed to keep the oxygen out of the drying space.
Among the advantages of EB equipment are the
following:
- The need for energy is 20-50 times less than in
thermal drying.
- Production speed is high.
- Thick layers of coating can be dried regardless of
the color shade applied.
- The end result is of high quality.
various moisture-absorbing salts or other substances.
The speed of drying is based on the air’s large capacity
to absorb the water contained in the paint.
9.10 UV-LED curing
The technology of UV-LED lamps is developing rapidly.
At the moment, UV-LED technology can be used to cure
both 100% and water-borne UV products in an inert, i.e.
an oxygen-free state.
The advantages of UV-LED technology include:
- Reduced need for energy (4 W/cm).
- Lamps may be turned on and off at short intervals –
the surface does not become heated.
Due to its properties, UV-LED technology is best
suited to the coating applications of heat-sensitive
materials. Major benefits can also be derived in terms of
energy savings. However, the speed of curing is in the
same category as traditional UV.
9.9 Dry-air equipment
A drying oven has been developed for water-borne
products, where quick drying is based on drying the air
in use. Air can be dried by cooling or using moistureabsorbing substances, for example. During cooling,
air dries while it releases water bound to it; the water
is collected in a separate drain basin. Cold and dry
air are heated to the required temperature of 35-50°C
and conducted into the drying oven. The oven may be
cyclic, but it is more often a chamber-type oven, where
the evaporation and drying conditions are monitored
automatically.
Drying is begun at the lower temperature of 35°C
combined with a fast rate of air circulation. Dry air has
the capacity to bind more moisture to itself than regular
room air, thereby enhancing evaporation and drying in
the oven. The moistened air is re-cooled, causing it to
release the bound moisture. The chamber method is a
closed process, i.e. the amount of air used for drying
remains the same throughout, allowing for major energy
savings. With the cold-drying method, the time required
to dry water-borne products package dry temperature
can be shortened to a mere 15 minutes.
Other dry-air drying ovens operate based on the
same principle. In these, the air can be dried with
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10. Testing the finished paint surface
T
he durability of the surfaces is tested using
standardized testing methods. Standardization aims
at making the testing methods of different countries
consistent, allowing the test results obtained in one
country’s testing laboratory to be utilised in other
countries as well. The goal of stress testing coatings is
to determine the durability of the product combinations
in normal use. Research methods have been developed
to resemble real-life circumstances and stress as closely
as possible.
In principle, surface testing can be divided into two
categories: exterior and interior testing. Exterior testing
aims at determining how well the product withstands
the external stresses imposed upon the object to which
it is applied. Interior testing determines how well the
paint combination withstands mechanical and chemical
stresses.
The primary exterior tests include QUV testing,
exterior field testing and microbiological tests. In
QUV testing, the paint surface is exposed to several
cycles of UV radiation and variations in humidity for
specified periods of time. After this, the paint surface is
evaluated in terms of gloss, durability of colour shade,
adhesion and elasticity. Similar tests are also conducted
subsequent to exterior field testing. In this way, the paint
combination’s resistance to long-term exterior stresses
can be evaluated. Microbiological tests are used to
determine the level of resistance of the paint surface to
fungal and mould organisms.
In testing interior products, the Möbelfakta-testing
method developed by the Swedish Möbelinstitutet is
used. This method tests resistance to water, alcohol,
coffee and acetone, as well as resistance to scratching,
grease, heat, moist heat and impact. Basic tests for
interior paint combinations also include the testing of
gloss, the durability of colour shade, and adhesion.
The Möbelfakta-tests aim to ensure that the chosen
combination endures normal interior stress.
In addition, there are specific testing methods
intended for special-purpose paint coatings. In the
ship-building industry, the special requirement for paint
combinations used for interior decorating is that they
do not spread fire. This is tested by heating the paint
surface to determine whether it ignites and spreads fire.
38
T I K K U R I L A
Specific fire-retardant paint should be used to ensure
more advanced fire protection. The purpose of fireretardant paint is to prevent the underlying material from
catching fire. Specific research methods have been
developed for these products as well – the surface is
heated and the underlying material is monitored for a
specified period of time to ensure it does not ignite.
Figure 39. Exterior field testing at Tikkurila Oy, Vantaa, Finland.
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I N D U S T R Y
11. Paint standards for
wooden surfaces
F
or testing paint surfaces suitable for outdoor use,
the current standard EN 927, Sections 1, 2, 3, 4
and 5 are complied with. Based on this standard, the
basic principles for testing paint surfaces suitable
for exterior painting are delineated. After one year of
external stress, treatment paint combinations must meet
the requirements set out by the standard in terms of the
following properties:
- Difference in specular gloss; EN ISO 2813
- Colour difference; ISO 7724-3
- Blistering (swellings in the surface layer); EN ISO
4628-2
- Flaking; EN ISO 4628-5
- Cracking, EN ISO 4628-4
- Chalking, paint peeling off; EN ISO 4628-6
- Growth of mould; EN ISO 4628-1
- Adhesion; EN 927-3, Appendix B
- General impression; EN ISO 4628-1
No similar pan-European interior standard is yet in
use. However, standard EN 15060 has been drawn up;
its purpose is to facilitate the selection of a treatment
combinations. The standard has not been taken into
active use in all countries. The testing of paint and
lacquer surfaces is still largely carried out according
to the Swedish Möbelfakta methods. In its quality
standards, IKEA makes reference to research methods
similar to those of Möbelfakta, thereby making these
methods widely spread.
Based on Möbelfakta, the resistance of the coating is
examined in terms of the following properties:
- Resistance to water; EN 12720
- Resistance to grease; EN 12720
- Resistance to grease and scratching; SS 83 91 22
- Resistance to scratching; SS 83 91 17
- Resistance to alcohol; EN 12720
- Resistance to dry heat; EN 12722
- Resistance to wet heat; EN 12721
- Resistance to water at the borders; SS 83 91 20
- Resistance to sweat; NS 8061
Figure 40. Tikkurila archives.
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39
12. Health and safety at work;
Environmental protection
12.1 Health and safety within
the paint industry
complemented by R-phrases indicating hazard and
S-phrases indicating general safety measures. Industrial
paints may fall into the following hazard categories:
A large portion of industrial paint products contain
substances that are hazardous to health and may cause
injury to the user, if he or she is not aware of the product’s
safety risks. The warnings on the product packaging
label, as well as safety data sheet informs paint-product
users of the possible health hazards posed by the use
of the product, as well as the protective measures to be
taken in order to avoid harmful effects.
The composition of paint products and their label
markings are influenced by a range of different national
and EU region regulation and directives. Among the EU
provisions are the REACH-regulation (1907/2006), the
Dangerous Substances Directive (67/548/EEC) and the
Dangerous Preparation Directive (1999/45/EU). At the
moment, extensive legislative changes are underway;
each operator must be aware of them inasmuch as they
concern him or her.
The operator must always exercise sufficient
diligence and caution in order to prevent health and
environmental hazards. Product users must be aware of
the risks involved. In addition, the operator must opt for
the least hazardous product of the alternatives available.
Moreover, the manufacturer, importer, distributor or other
operator, who releases the chemical onto the market,
must draw up a safety data sheet pertaining to the
product.
Basic information regarding the hazards involved in
using paint products, as well as the necessary protective
measures – such as using a respiratory protective mask,
the need for skin protection, etc. – are included on the
packaging label. Further information is available in safety
data sheets and product data sheets.
12.2.1 Reactive products
Flammable
Highly flammable
12.2.2 Products which are hazardous
to health
Harmful
Irritant
Corrosive
Toxic
12.2.3 Products which are hazardous to the
environment
Dangerous for the
environment
12.2 Package markings
The warnings printed on the paint-product label provide
information on the risks involved in using the product.
Reactivity, and health and environmental hazard symbols
are shown in the warning section of the label. These are
40
Oxidizing
T I K K U R I L A
On the warning symbol, a black image always appears
against an orange background.
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I N D U S T R Y
12.3 Safety data sheets
More detailed information on the composition of the
paint product, its properties, as well as the hazards
involved in using the product, and the necessary
protective measures are displayed in the safety data
sheet concerning the product. All safety data sheets
pertaining to products that contain substances that are
hazardous to health and which are used on the work site
must be kept in a place where workers can peruse them
at any time.
According to the current rules and regulations, the safety
data sheet contains the following information:
1. Identification of the substance/preparation and
of the company/undertaking
2. Hazards identification
3. Composition/information on ingredients.
4. First aid measures
5. Fire-fighting measures
6. Accidental release measures
7. Handling and storage
8. Exposure controls/personal protection
9. Physical and chemical properties
10. Stability and reactivity
11. Toxicological information
12. Ecological information
13. Disposal considerations
14. Transport information
15. Regulatory information
16. Other information
Old safety data sheets dated prior to June 1, 2007,
whose headings differ from the above, may be delivered
up until December 1, 2010, provided there are no
significant changes in the product properties prior to
that date.
However, the safety data sheets may not express any
views concerning the special circumstances at the work
site. Air conditioning, the safety of the work methods
used and the adequacy of the protective measures must
always be considered on a case-by-case basis at each
work site. By adhering to the instructions given and using
common sense, all paint products may be safely used.
Eventually – as a result of what are known as exposure
scenarios conducted on the basis of registering the
substances used and estimating the potential hazard
according to the REACH regulation – an opinion will
be expressed regarding risk-management measures
as well. However, several more years are expected
I N D U S T R I A L
to elapse before these types of extended safety data
sheets become available.
12.4 Fire safety
The solvent vapour from flammable liquids or spraying
mist may – when combined with oxygen in the right ratio
– form a flammable mixture. The properties of the vapour
from different solvents vary to some degree; however,
in terms of fire safety, it is critically important that the
protection category of spray-painting equipment, pumps
and other like electrical equipment is appropriate to the
ignitability and explosiveness properties of the paint
products in use. All work that produces fire sparks, such
as welding and flame cutting, etc. is absolutely forbidden
in the vicinity of painting work.
The mixture of solvent vapour and oxygen may be
ignited by an open flame, welding spark, or another type
of spark, for example one generated by the discharge of
electricity. For this reason, all spray-painting equipment
and tools must be absolutely well grounded in order to
allow for the safe discharge of the electricity generated
by the flow of liquids etc.
Basic information on the flammable and explosive
properties of paint products is contained in the product
data sheets and safety data sheets. In order to maintain
personnel and fire safety, continuing education, guidance
and counselling of operations personnel in terms of
appropriate and careful use of the products and working
methods is of primary importance.
On the other hand, safety also requires careful
monitoring and ongoing process development in order
to eliminate hazardous properties. Using water-borne
and solvent-free products will significantly reduce the
paint shop’s fire-safety risks.
Porous cleaning waste, spray mist and other
substances containing a binder that dries by oxidizing
– such as linseed oil, lacquer or alkyd resin – pose a
particular fire safety risk. This type of waste may selfignite, if the heat generated while the binder dries is not
released into the environment. This may happen, for
instance, when cleaning paint dust from a dry spraying
booth immediately after spraying and collecting a large
mass of waste in a dry container. In such a case, the
reaction continues, generating heat which may ignite
the burning materials mixed in with the waste. This can
be prevented by adding water so that it is level with the
surface of the waste. This type of material must be stored
outside the plant facilities at all times, especially during
periods when the facility is not being monitored.
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12.5 Using paints
National laws and decrees and pan-European decrees
limit and guide the use of certain finishing materials.
VOC directives are among the provisions that govern
painting.
Two VOC directives are currently in force: 1999/13/
EU, which limits the use of solvents in certain industrial
operations; and 2004/42/EU, which defines the maximum
allowable VOC content of ready-to-use paint mixtures
in products intended to be used for buildings. Of the
two, industrial operations are more restricted by the
first-mentioned directive, known as the industrial VOC
directive.
The fundamental purpose of new provisions – such
as the biocide directive, the REACH regulation and
global harmonization of classification of chemicals
(the CLP regulation) – is to limit the use of hazardous
substances and protect the environment, as well as
the manufacturers and users - including the end users
- of chemical substances. All directives, laws and
regulations will impact the use of chemicals in paints. At
the same time, the operational properties of paints will
change, requiring the further development of the related
operating conditions as well.
More comprehensive information on labour protection
and environmental protection is available in Industrial
Coatings’ environmental and safety guide for industrial
finishing. Additionally, painting operations are guided
by e.g. Finnish Standard SFS 3358: Painting; Facilities,
operation, maintenance, fire-fighting equipment and
guidance on classification of hazardous areas.
42
T I K K U R I L A
O Y
I N D U S T R Y
13. Objects to be coated and
recommended paint
combinations
T
he finishing combination is always determined by
the company’s product selection and equipment
available for use. In addition, the wood, and the desired
appearance and quality level may influence the choice
of the most commonly used finishing alternatives for
different objects.
achieve a sufficiently smooth end result, the application
quantities in the oven must be increased. This requires
more elastic paints that can withstand more extensive
applications without cracking. Diccoplast Elastic paints
are recommended.
The following are among the paint combinations
recommended for frame structures, where the substrate
is made of e.g. UV or alkyd-puttied chipboard.
13.1 Furniture
Acid catalysed paints and lacquers have established
themselves as the finishing materials of choice for
furniture. Concern for the environment has also brought
water-borne and UV curing products and combinations
thereof to the fore.
Typical paint combinations include the following:
Primer
Primer
Diccoplast Primer 1-2x
Diccoplast 1x
Akvi Primer 2x
Akvi Top 1x
Akvi Primer 1-2x
Luminol 1x
When the door substrate is made of MDF board, the
right paint combination is the following:
Topcoat
Diccoplast Primer 1-2x
Diccoplast 1-2x
Novipur Primer HS 1-2x
Novipur 30,100 1-2x
Akvi Primer 1-2x
Akvi Top 1x
Akvi Primer 1-2x
Luminol 1x
Uvicol Primer 2-3x
Uvinol 1-2x
Primer
Diccoplast 1x
Akvi Primer 1-2x
Luminol 1x
When the door substrate is solid/veneered birch, pine or
hardwood, the appropriate coating combination is the
following:
Sealer
Merit or Merit Topas 1x
Top lacquer
Merit Sealer 1-2x
Merit or Merit Topas 1-2x
Merit Zircon Extra 1-2x (1-comp.)
Merit Zircon Extra 1x (1-comp.)
Novipur Sealer Tix 1-2x
Novipur Clear 1x
Akvilac Topas tai Akvilac 1x
Akvilac Topas or Akvilac 1x
Luminol 1x
Luminol 1x
Uvinol Sealer HB 1-2x
Uvinol 1-2x (flat object lines)
Uvinol Sealer HB 1-2x
Merit Topas or Merit 30 1-2x
Topcoat
Diccoplast Primer 1-2x
Finishing combinations suitable for lacquering
include the following:
Sealer
Topcoat
Top lacquer
Merit or Merit Topas 1x
Merit Sealer 1-2x
Merit top lacquer 1-2x
Akvilac Topas or Akvilac 1x
Akvilac Topas or Akvilac 1x
Luminol 1x
Luminol 1x
The degree of whiteness of a solvent-borne lacquer
can be adjusted by adding 0.1-0.5% white Temaspeed
Premium colorant. The degree of whiteness of a waterborne lacquer can be adjusted with white Temaspeed
Fonte or Symphony/Avatint colorant.
For stained surfaces:
13.2 Fixtures
The use of MDF board as a material for doors has grown
significantly. Since MDF board is easy to process by
milling, for example, it is commonly used in panel doors
and other grooved furniture. In the milled parts, the
denser surface layer has been removed, making the
paintable surfaces in the grooves quite porous. In order to
I N D U S T R I A L
Sealer
Top lacquer
Merit or Merit Topas 1x
Merit or Merit Topas 1x
Akvilac Topas or Akvilac 1x
Akvilac Topas or Akvilac 1x
Luminol 1x
Luminol 1x
W O O D
F I N I S H I N G
43
13.3 Interior doors
Interior doors are a typical object for fast-drying products.
Water-borne or water-borne UV products are gaining
ground as coating lines are rebuilt to accommodate
water-borne products.
Solvent-borne paint combinations:
Primer
Topcoat
Diccoplast Primer 1-2x
Diccoplast 1x
Dicco Surf 1-2x
Dicco Astral 1x
Water-borne and water-borne UV paint combinations:
Primer
Topcoat
Akvi Primer 1-2x
Akvi Top 1x
Akvi Primer 1-2x
Luminol 1x
day, dark-painted wood surfaces reach temperatures as
high as 70°C, while the temperature of a white surface
remains at approximately 40°C.
Wood swells and shrinks as its moisture content
level varies, causing considerable stress to the paint
surface. The paint must withstand the variations in the
wood’s dimensions. An effort is made to minimize water
absorption through the finishing treatment of the wood
surface. Special attention must be paid to protecting
the joining points and end surfaces. In wood structures,
long-term humidity of over 20% must be avoided, as this
would expose the wood to fungal spores.
Tikkurila has developed a new generation waterborne Pinja Flex paint combination for painting windows
and exterior doors.
The advantages of this combination include the
following:
Lacquer combinations:
Sealer
Top lacquer
Merit or Merit Topas 1x
Merit or Merit Topas 1x
Merit Sealer 1-2x
Merit or Merit Topas 1x
Akvilac Line 25 1x
Akvilac Line 25 1x
Luminol 1x
Luminol 1x
13.4 Lamella parquets
For lacquering parquet floors, highly durable UV
hardening lacquers are best. These are ordinarily
applied in roller coater lines. Parquet lacquers are
generally developed according to customer and linespecific requirements. The various requirements include
adhesion, abrasion resistance, sandability and surface
appearance. For maintenance lacquering of UV lacquer,
the most common parquet lacquers – such as Parketti
Ässä (Parquet Ace) – are appropriate when preceded by
a good sanding. Uvipar parquet lacquers are developed
to suit specific lines, and are used for priming and top
lacquering.
••These products fulfil future labour protection and
environmental protection provisions. The products are
water-borne and thus minimize solvent emissions.
••The water-borne window-paint film is permanently
elastic, living well alongside the wood up to dimension
changes of 7-8%.
••The water-borne window-paint film allows water vapour
penetration; yet it is water repellent.
••The water-borne paint film is exceedingly weatherresistant. The paint surface withstands long-term outdoor
stress without losing its gloss or colour shade.
Pinja Flex paints can be sprayed using all common
spraying methods. A completely dry surface that can
withstand severe stacking conditions is achieved by
drying the topcoat in 35-45°C for at least 1-2 hours, and
then cooling the surfaces before stacking. Pinja Flex is
part of the Temaspeed Fonte tinting system.
Recommended for painting and glazing:
13.5 Windows and exterior doors
The surface structure of wood changes quickly when
affected by sun light. For wood, painting provides the best
protection against sun light. White pigments efficiently
light and prevent them from penetrating the wood
surface. In painting, white shades are recommended,
as they keep the wood’s surface temperature as low as
is possible. High temperatures in wet wood promote the
growth of various types of fungi. On a warm summer
44
T I K K U R I L A
Primer
Topcoat
Pinja Flex Primer 1-2x
Pinja Flex 1-2x
Dicco Flex 1-2x
Dicco Flex 1-2x
Diccodur Primer 1-2x
Dicco Flex 1x
Glazing
Lacquering
Pinjasol 1x
Merit Jahti 2-3x
Pinja Wood Stain
Pinjalac Solid 1-3x
O Y
I N D U S T R Y
• Unprotected timber is stored away from rain and
drain water.
Manufacturing and pre-treating done inside a warm
factory hall, instead of outdoors, clearly improve the
painting quality and prolong the product’s service life.
13.6 Mouldings, frames and panels
Glazing
Lacquering
Pinja Color 1x
Pinjalac Solid 1-3x
Pinja Wood Stain 1x
Pinjalac Solid 1-3x
The vigorous rate of construction in recent years has seen
the emergence of several companies that specialize
in treating elongated timber. Most commonly, these
companies use a linear spraying line where a thixotropic
paint is best suited for this application method.
Suitable paint and lacquer combinations:
Primer
Topcoat
Diccoplast Primer Tix 1-2x
Diccoplast Tix 1x
Akvi Primer Tix 1-2x
Akvi Top 1x
Akvi Primer Tix 1-2x
Luminol 1x
Sealer
Top lacquer
Merit or Merit Topas 1x
Merit or Merit Topas 1x
Akvilac Line 25 1x
Akvilac Line 25 1x
Luminol 1x
Luminol 1x
13.7 Exterior claddings
The starting point for improving the weather resistance
of wood is keeping the level of humidity in the structures
as low and even as possible. This can be promoted by
the following means:
• Eaves and visors.
• Appropriate eave cutters and down pipes.
• A tall enough socle.
• Correct placement of the window.
• Adequate inclination of the window lead.
• Rainproof structure.
• Dryable facade.
• No water- and humidity-collecting surfaces, seams
or grooves.
• The windows are tightly glassed.
• Drip beaks at the ends of panels.
• Finishing of cross-cut surfaces.
• Solid fastening; avoid cracking the wood when
nailing.
• Appendages are appropriately fastened to the
facades.
• Priming is done sufficiently soon, preferably before
installation.
I N D U S T R I A L
The coating result is affected by the choice of
substrate:
a) An effort should be made to use heartwood as a raw
material if pine is used.
• Surface
panels
always
present
problems
(knots and annual rings).
• Spruce
is
preferable
to
pine.
b) Dry sawing is done by band sawing; the feeding
speed is moderate (< 50 m/min).
• Wood structure has an impact.
c) Depending on the product, profiling – or at a minimum,
the slight elimination of corners, resulting in a flawless
surface.
d) In connection with planing, a procedure known as
grinding may be used. This will give the surface a slight
resemblance to a round-cut surface and then adhesion
is excellent.
e) Moisture content of the panels must be under 20%.
f) The cutting time of the tree affects fungal growth.
g) Water-storage of wood prevents blue stain growth,
but enhances bacterial activity.
In order to facilitate painting work and to reduce
costs, several different types of finishing equipment
have been developed. The use of industrially finished
timber yields undeniable benefits:
• The first treatment – which is critically important in
terms of the end result – takes place under controlled
circumstances.
• Timber is always treated on at least three sides,
ensuring that, for example, tongue-and-groove
   surface-sawn
timber
will
receive
finishing
treatment throughout.
• Timber can easily be treated with wood stain.
• Knot lacquering can be done quickly before priming.
• Nowadays, construction work is performed year round
and painted wood elements are installed at a time when
finishing cannot be done outdoors.
Tikkurila Oy has developed a water-borne finishing
combination, Pinja 2+, specifically for industrial priming.
Pinja Oil is a water-borne primer that penetrates deep
into the wood, providing excellent protection against
blue stain and mould. Pinja Pro Primer is a water-borne
W O O D
F I N I S H I N G
45
primer that can be used as a primer for both oil and
water-borne paints, making it both user and environment
friendly. Pinja Pro Primer is available in the Avatint tinting
system. It suits most painting equipment; spray and
vacuum equipment are among them.
We recommend the following paint combination:
Primer
Pinja Oil 1x +
Pinja Pro Primer 1x
Topcoat
1x with any of Tikkurila’s most
common exterior house paints
(e.g. Pika-Teho, Ultra, Vinha, Teho)
13.10 Fibreboard
Much attention is paid these days to environmental and
air protection. The authorities have also tightened the
rules and regulations considerably. In order to reduce
environmental hazards, an effort should be made to use
paint combinations that are either completely solvent
free or contain less solvents. Tikkurila’s water-borne
Akvi products – either by themselves or combined
with Diccoplast acid catalysed paint or UV hardening
Uvinol products – are the modern paint combinations for
fibreboard.
A. Smooth board
13.8 Beams
Primer
Wood beams must have excellent resistance to humidity.
Suitable coatings include the following:
Akvi Board, according to the line
Lacquering
Akvi Board, according to the line,
or Uvinol Board, according to the
line
B. Moulded board
Coating
Pinja Color 1x + Pinjalac Solid 2-3x
Primer
Pinjasol 1x + Merit Jahti urethane lacquer 2x
Akvi Primer 1-2x
13.9 Plywood
Painted plywood is used primarily for objects that are
subjected to extreme stress. Therefore, the paint must
perform exceedingly well in terms of elasticity, strength,
mould resistance and ability to withstand steam cleaning.
Epoxy paints and the polyurethane paints in the Diccodur
series are best suited for this purpose.
These coatings are among those used for finishing
plywood:
Lacquering
Akvilac Topas or Akvilac Top lacquer 2-3x
Edge painting
Akvi RMS 2-3x
Merit Top lacquer 1-3x
Uvinol -lacquer, normally 4-5 roller
coaters
46
T I K K U R I L A
O Y
I N D U S T R Y
Topcoat
Diccoplast, Akvi Top or an alkyd
paint
14. Coating costs and cost calculations
T
he total industrial finishing costs per square metre
consist of several different cost factors. Significant
among these are the costs of facilities, equipment and
personnel (coating, goods handling and quality-control
personnel). Additionally, there are costs associated with
energy and the environment (waste-water purification,
waste management, etc.). Often, repair costs associated
with finishing are left unnoticed. These costs may arise
from processing the wood, the finishing method itself,
a poorly performing paint combination or intermediate
sanding.
Each company has its own cost structure. It is
interesting to note, however, that the choice of an
appropriate coating can affect several of these costs.
The choice of appropriate coating equipment also has a
significant impact on the total costs of finishing.
The price of the coating, which is often used when
comparing different coatings, is only a fraction of the total
costs of coating. In fact, the price per litre of industrial
coating says very little about what the finished surface
will cost. The price of the coating in the can is a different
matter to the price of the coating on the finished surface.
Therefore, the per-litre price in itself is not an appropriate
basis for choosing an industrial coating.
In the wood industry, the amount applied to flat objects
is measured using scales. The piece to be measured is
fed normally through the painting line; before drying, the
amount of paint applied on the sample piece is measured
(g/area of measuring sample). It is recommended that
the size of the measuring piece be kept to one that
is easily converted into square metres, for example,
50 mm x 200 mm. Alternatively, a table can be prepared
based on the area of a standard piece – placing the
table on the wall will allow you to quickly determine to
the gram the amount applied in g/m2.
Indirectly, the measuring value g/m2 may be used
when calculating coating costs. When evaluating the
quality and durability properties of the surface, the g/m2
value is instructional only. The properties of the painted
surface are determined according to the thickness
of the film. The specific gravity (kg/l) of paint varies
according to how many raw materials with high specific
gravity the paint contains. These heavy ingredients do
not necessarily improve the quality properties of the
finished surface. When comparing different paints, it is
recommended that the g/m2 measuring result always be
I N D U S T R I A L
converted into film thickness (µm).
In a hanging conveyor line, measuring the amount
applied is usually done with a wet-film comb.
The test
sample is fed into the painting device and after painting
the amount of wet paint applied is measured with a wetfilm comb. The measurement result is indicated by that
part of the gauge’s tooth that still becomes wet from the
applied compound. The thickness of the dry film can be
calculated when the solids content of the mixture used
is known in terms of percent by volume.
Below are a few examples of different cost and filmthickness calculations. The samples show how coating
costs are calculated. The prices, weights, volumes and
mixture ratios used are examples only.
Table 4. One-component solvent-borne lacquer.
Mixture
Volume
(l)
Weight
( kg )
Solids
( kg )
Price
(€)
Lacquer
10
9,5
2,85
65,00
Thinner
2
1,7
-
7,00
Total
12
11,20
2,85
72,00
Price of lacquer
€ 6.50/l
Solids content
30 weight %
Specific gravity of lacquer
0.95 kg/l
Price of thinner
€ 3.50/l
Specific gravity of thinner
0.85 kg/l
Mixture ratio (in volumes)
100 parts lacquer
20 parts thinner
In the Volume column, all ingredients to be mixed
in the lacquer mixture are added up. The weight of the
lacquer is calculated according to the formula; volume
x specific gravity = 10 l x 0.95 kg/l = 9.5 kg. In terms
of lacquer, the amount of solids is: weight of lacquer x
solids content (as percentage of weight) = 9.5 kg x 0.30
= 2.85 kg.
The price of the lacquer is calculated directly from the
price per litre:
Volume x price of lacquer = 10 l x 6,50 €/l= 65,00 €.
Price per litre of the mixture 72,00 € / 12 l = 6,00 €/l
Price per kilo of the mixture 72,00 € / 11,2 kg = 6,42 €/kg
Price per kilo of dry film 72,00 € / 2,85 kg = 25,26 €/kg
W O O D
F I N I S H I N G
47
Materials cost of lacquer:
If the amount applied is 120 g/m2 of wet compound, the
price of the film = 120 g/m2 x 6.42 €/kg =
0.12 kg/m2 x 2 €/kg = 0.77€/m2.
Solids content of the mixture as percentage of weight
(2.85 kg / 11.2 kg) x 100 = 25.45 weight %.
What are the material costs of paint, if the desired goal is
50 µm dry film with one application?
Solid content of mixture:
(20 x 0.64 + 4 x 0.50 + 2 x 0.00 %) / 26 = 57 volume %.
The specific gravity of the paint mixture is 30.62 kg / 26 l
= 1.18 kg/l.
If you would like to know how thick a coat of paint can
be achieved with one application, you need to know – in
addition to the preceding information – the solids content of
the lacquer as percentage by volume. You will also need to
calculate the specific gravity of the lacquer mixture.
The specific gravity of the lacquer compound is
11.2 kg / 12 l = 0,93 kg/l.
- Wet application is derived by dividing the desired
amount of application by the solids content of the paint
mixture (volume %) = 50 µm / 0.57 = 88 µm of wet
mixture.
If the solids content of the lacquer is 26, the volume %
can be calculated as follows:
- Wet application = 0.12 kg/m2 / 0.93 kg/l =
0.129 l/m2, since 1 l = 1 dm3, the result is
 0.129 dm3 / 100 dm2 = 0.00129 dm = 129 µm.
- Consequently, the thickness of the dry film is
129 µm x 26 volume % = 34 µm.
Even though the above examples concern different
types of products, it is evident from them that low cost
per litre in itself does not entail lower materials cost. The
price per litre of the paint product is clearly higher than
that of lacquer; in spite of this, the materials cost per
square metre is less, and at the same time, an even
thicker paint film is achieved using paint.
In the above calculations, the costs for only one layer
are calculated. In normal finishing work, the products are
painted at least twice, thereby approximately doubling
the material costs of the total quantities of paint and
lacquer used. Also, only the share of the paint to be
applied to the surface of the object is calculated, i.e. it is
a theoretical calculation of cost.
On roller coater lines, and when curtain coating
machines are used, efficiency as high as 90-99% is
achieved, while in some finishing methods, the efficiency
may drop as low as 30-40% – for example, in spraying
chairs. If the share of loss is not taken into consideration,
a large portion of material costs will be excluded. It is
important to assess loss when the costs of different
finishing methods are compared before selecting a
method to be used.
In practice, the chemical reaction between different
binders and the hardeners used also impact the film
thickness. The result is that as the film dries, it also
shrinks. Shrinkage can amount to as much as 10% of
the layer thickness.
Table 5. Two-component solvent-borne paint.
Mixture
Volume
(l)
Weight
( kg )
Solids
( kg )
Price
(€)
Paint
20
25,00
17,50
150,00
Hardener
4
3.88
2,17
35,60
Thinner
2
1,74
-
7,00
Total
26
30,62
19,67
192,60
Price of paint
7.50 €/l
Solids content
70 weight %, 64 volume %
Specific gravity
1.25 kg/l
Price of hardener 8.90 €/l
Solid content
56 weight %, 50 volume %
Specific gravity
0.97 kg/l
Price of thinner
3.50 €/l
Solids content
0 weight %, 0 volume %
Specific gravity
0.87 kg/l
Mixture ratios
100 parts paint
20 parts hardener
10 parts thinner
Mixture price per litre 192,60 € / 26
= 7,41 €/l
Mixture price per kg 192,60 € / 30,62 = 6,29 €/kg
Dry film price per kg 192,60 € / 19,67 = 9,79 €/kg
48
T I K K U R I L A
- The materials cost is derived as follows: 88 µm x 7.41 €/l
= 88 x 10-6 m x 7.41 €/l x 10-3 m3 = 0.65 €/m2.
The wood material used also makes a considerable
impact on paint-layer thickness. Paint compounds are
also absorbed into the wood also in order to achieve
the best possible adhesion. Of course, the paint layers
absorbed into the wood of course reduce the thickness
O Y
I N D U S T R Y
of the paint film. If, afterwards, you wish to measure – with
a microscope or an ultrasound device – the thickness of
the paint film achieved, you will discover that it varies
a great deal in different places. This is because the
wood surface is never even; the cell structure of oak, for
example, may have differences exceeding 1 mm.
It is best, then, to check the material costs and total
costs of finishing throughout the process. By following the
current trends in technical development, considerable
savings can be achieved by changing materials or
investing in equipment.
I N D U S T R I A L
W O O D
F I N I S H I N G
49
15. General instructions before finishing
B
efore starting finishing work, the product data sheets
and Safety Data Sheet should always be carefully
reviewed, especially when using a product for the first
time.
Add the right amount of the right hardener
It is important to follow the recommendations for twocomponent products. Too little hardener slows down the
drying, or in the case of polyurethane paints, may result
in too soft an end result. Too much hardener makes the
coating surface too hard and causes it to crack easily.
In acid catalysed products, excess hardener will usually
rise to the surface as mildew.
Mix well
Pigments and matting agents may stratify while in
storage. Mix the bottom sediment well; otherwise gloss
and shade variations may occur. Lacquers may also
incur “invisible” bottom sediment; in such a case, the
product must be mixed well in order to achieve an
even gloss. Wait a moment before applying the thinned
coating to allow it to “settle”; this will help avoid foaming
and other problems after application.
When using acid catalysed products, mix the coating
in a separate container
The two-component coating, which contains the
hardener, may turn a reddish-brown colour when stored
in the container it was delivered in. It is best to store the
mixture in a container made of polyethene or stainless
steel.
Mixing two-component products
Two-component products must also be mixed well before
adding the hardener. This ensures that the hardener is
evenly mixed with the paint.
• Mix the opened can before adding the hardener or
thinner. Mixing the paint/lacquer is particularly important
when using thixotropic products. By mixing, “false
thixotropy” is removed, and excessive dilution of the
product is avoided.
• Check also to ensure the paint temperature
corresponds to the temperature in the painting space.
By checking the temperature the appropriate viscosity
level is ascertained from the start, and variations in
thinner quantity can be avoided.
• Add the hardener directly into the paint and begin
mixing. PLEASE NOTE! Avoid adding the thinner at the
same time, as doing so may hinder the mixing of the
paint and the hardener.
• Mix thoroughly until the hardener has completely
disappeared from the paint surface. The mixing time
generally varies between 30-60 seconds. If possible,
mix by machine, as this will ensure the mixing is even
and shorten the required mixing time.
• Add the right amount of thinner and mix in well with the
paint mixture. The mixing time may now be cut in half.
• Wait for approximately 10-15 minutes before mixing
the paint mixture once again, and check to ascertain the
viscosity is correct – the paint mixture is now ready for
use.
50
T I K K U R I L A
Check to make sure that the substrate is clean
Oil-, wax- and silicone-based impurities cause wetting
problems or glossy blotches and hinder drying.
Check to make sure that the sealer or primer is not
sanded through at any point
Sanding through may cause absorption, grain raising
and an uneven gloss. Determine the coating quality
according to the requirements of the substrate, coating
equipment and the finished surface.
Check the viscosity
Viscosity should be measured at 20-25°C. If the coating
has been stored in cold temperatures, more thinner
will need to be added and the mixture will acquire
inappropriate properties. Excessive thinning reduces
the solid content of the mixture; obtaining sufficient
thickness of the dry film may be impossible.
Check the shade
Prior to finishing large pieces, the shade should be
checked on smaller test samples. Especially when
staining large surfaces, a fresh shade sample should be
available for use.
O Y
I N D U S T R Y
Check the gloss
Check the gloss when beginning finishing where
a product or a new batch is used for the first time.
Inadequate mixing prior to use may cause differences in
gloss in the finished surface.
Use the recommended thinner
The coating requires a certain amount of thinning
(solvents) in order to function well with different
application devices. Different thinners may be used
with different paints and lacquers; their appropriateness
should be checked before use. Using general thinners in
special circumstances is possible – they may, however,
cause disturbances in drying, or variances in shade or
gloss.
Determine the amount to be applied
The amount to be applied is calculated according to
the desired end result, depending on coverage, type of
wood and other requirements in terms of appearance. By
coating a test sample, you will obtain an understanding
of how much coating is required on the substrate.
In order to determine the amount to be applied you
may simply test-paint a sheet of paper 20 cm x 50 cm
(=0.1m2) or 20 cm x 25 cm (0.05 m2) in size. A sheet
of paper can be attached to a flat surface using nails
that have been hammered through a board from its rear
side. Weigh the sheet of paper before and after coating.
The amount to be applied is indicated in grams in terms
of the difference in weight before and after coating by
multiplying the result by 10 or 20.
In a curtain coating machine, both the viscosity and
the amount applied must checked. This should be done
when starting, when adding coating and at least once an
hour throughout the work process. Automatic viscosity
adjusting devices that can be attached to both curtain
coater and roller coater machines are available on the
market.
Check the pot life
When hardener is added to two-component coating,
hardening begins. The time required for doubling the
viscosity is known as pot life. Pot life varies from a few
hours to several days, depending on the product. For this
reason, pot life should always be ascertained from the
product data sheet. The excess product may be usable
on the next day. In that case, it should be used to obtain
a new mixture that consists of one part of the old mixture
and two parts of the new one. In this way, the coating will
maintain its original properties, including gloss.
Read and follow the warnings!
I N D U S T R I A L
W O O D
F I N I S H I N G
51
16. Some of the most common
concepts in painting
T
his guide, as well as paint-product labels, product
data sheets and Safety Data Sheet contain some
special terminology. This glossary defines some of the
principal concepts in order to facilitate the proper use of
paint products and to enable the painter to complete the
work without difficulty.
Density
Density indicates the weight of one litre of paint at
23°C. The unit of density is kg/l as defined according to
the standard SFS 3635.
Dispersion
Dispersion is the name used for effective mixing in paint
manufacturing. Paints are manufactured in a dissolver.
At the end of its rotating axle is a sheet metal gear used
to generate sufficient shear force to grind the pigments
that have been added to the paint.
One-component
The paint dries either physically, or an internal hardener
has been added to it during the manufacturing phase.
There is no need to add hardener during the mixing
phase.
Dry-film thickness
The thickness of the coating remaining on the substrate
(µm) when the solvents have evaporated and the film
has hardened.
Two-component
This is a paint to which the amount of hardener indicated
in the product data sheet must always be added.
With adding the hardener the paint dries and forms a
cohesive surface that protects the substrate against
stress. Without the hardener, drying does not take place;
the surface remains sticky, and does not acquire the
required resistance properties.
Evaporation time
Please see Flash-off time below.
Filling capacity
Filling capacity refers to the capacity of the paint product
to cover up the unevenness of the substrate. This is
dependent on the paint’s solids content and tendency
to be absorbed into the substrate. If the paint has poor
filling capacity, several layers must be applied in order
to obtain the desired surface evenness.
Acid catalysed lacquer or paint
A paint product, whose binders consist of various resins
which harden due to the influence of catalytic acid. The
reaction starts after the acid has been added and the pH
level drops. The acid does not react with the binder.
Acrylate, acrylate polymer
Acrylate is a binder that is commonly used in waterborne products, among others.
Flash-off time
Subsequent to painting and prior to drying at an elevated
temperature, the solvents must be evaporated from the
paint film. The time it takes for the solvents to evaporate
is called Flash-off time. Evaporation time depends on
the type of paint, its solvent composition, film thickness,
temperature and ventilation. Without sufficient flashoff time, the paint surface may boil while in the drying
zone.
Coverage
The capacity of the paint film to cover the differences
between the dark and light areas of the substrate is
known as coverage. Coverage is estimated by applying
a paint film of a certain thickness on what is known as
hiding power paper, on which a black check pattern
is printed against a white background. Coverage is
assessed by comparing the paint’s capacity to cover the
black-and-white check pattern on the paper.
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T I K K U R I L A
Gloss level
The gloss level indicates the capacity of the paint
surface to reflect light. Gloss can be defined in terms
of different angles of reflection. Generally, the wood
industry uses the 60° reflection angle. Measurement is
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I N D U S T R Y
performed according to standard SFS 3632/60°. Other
commonly used measuring angles are 20° and 85°. The
20° measuring angle can be used when measuring gloss
levels over 70; the 85° angle is used when measuring
gloss levels under 10.
Gloss level
90
80
60
35
10
5
≤x
≤x
≤x
≤x
≤x
≤x
x
< 90
< 80 < 60 < 35 < 10 <5
Nominal marking
Full gloss
High gloss
Gloss
Semi gloss
Semi matt
Matt
Full matt
Pulverized products (Powder coating)
Paint which is sprayed in pulverized form; it forms a paint
film during thermal heating.
Pot life
The pot life of two-component products is the time required
for the viscosity of a ready-to-use paint mixture to double
(e.g. 20 s/DIN 4 cup – 40 s/DIN 4 cup).
Solids content
The solids content of paint products can be indicated
either as percentage by volume or percentage by weight.
As percentage by volume, the solids content signifies
the ratio of the paint’s non-volatile ingredients to the total
volume of the paint quantity. The solids content is defined
according to SFS standard 3637.
Hardener
An ingredient added to the paint portion of two-component
paints that causes a hardening reaction. The choice
of hardener can be used to control the application and
drying properties of paint.
High-solid products
The solids content of high-solid products is higher at the
application phase than that of normal paints. The solids
content of high-solid paints is over 70% by volume while
that of high-solid lacquers is over 55% by volume.
Metamerism
The apparent change in colour shade due to a change
in lighting circumstances, for example when moving from
outdoors to fluorescent lighting.
Nitrocellulose
Cellulose polymer (binder) is made from cotton and the
cellulose in wood. Lacquers are made by dissolving
nitrocellulose in solvents. Another name for nitrosellulose
is sellulose nitrate
Operating viscosity
Operating viscosity indicates the paint product’s
viscosity after the hardener and the thinner have
been added. The paint mixture must have the right
operating viscosity in order to avoid possible defects.
I N D U S T R I A L
The solids content can also be expressed as percentage
by weight. This refers to the ratio of the weight of the nonvolatile ingredients to the total weight of the paint quantity.
When application quantities and film thicknesses are
defined, the solids content of the paint mixture must be
calculated based on the solids content of the individual
ingredients.
Shelf life
Shelf life is the period of time during which the coating
material retains its usability in the original, sealed containers
in the proper transport and storage conditions; when the
temperature varies between + 3°C and +30°C.
Solvent
A component in solvent-borne paints, whose purpose is
to dissolve the solid binders (resins) and polymers and to
reduce their viscosity.
Solvent-free paints
Paints that do not contain solvents; for example solventfree epoxies and 100 % UV products.
Structural surface
A structural surface is a dried paint film that has been
made uneven and nodular by using additives. The paint
film is often matt and covers up the unevenness of the
substrate.
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53
Theoretical coverage
The theoretical coverage of use is indicated by m2/l, and
can be calculated as percentage by volume based on
the solids content percentage (sc %) and the desired
thickness of the dry film (dft, micrometres µm). The
theoretical coverage of use is derived using the formula
10 * sc % /dft. For example, sc (by volumes) % = 65% and
dft = 60 µm. In such a case, the theoretical economy of use
is (10 * 65/60) = 10.8 m2/l. In actuality, the painting methods
and conditions, the shape of the object to be painted, the
surface quality and the painter’s occupational skill have
an impact on the actual coverage. The actual economy of
use is always smaller than the theoretical one.
Thinner
A thinner is a volatile fluid, solvent, water or a combination
thereof. It is used to dilute (decrease the viscosity of)
paint. Thinners evaporate from the paint surface during
the spraying and drying phase. The properties of thinners
are significant in terms of the application and drying
properties of paints.
Thixotropy
When the shear force increases – as is the case in a spray
nozzle – the paint reacts in such a way that its viscosity
drops momentarily and picks up again after a while. This
type of paint is easy to spray and will also even out; it does
not sag as readily on vertical surfaces.
Tinting system
A precise, quick and economical manufacturing method
of coloured paints suitable for products for which basic
paints exist. The tinting system consists of tinting pastes,
basic paints, tinting-formula files, a tinting machine and
mixer (shaker).
UV-drying
Paints and lacquers that are specially developed for UVdrying are dried with UV-lamp radiation. These products
attain the required chemical and mechanical properties
only subsequent to the UV-radiation.
UV-radiation
Short-wave radiation of light whose wave length is 180- 380
nanometres. UV-radiation is invisible to the human eye; it is
emitted by the sun and special lamps (UV-ovens).
Viscosity
In measuring viscosity, the DIN (SFS EN 456) measuring
method is commonly used. Measuring viscosity is based
on the substance being fluid enough to run through a small
hole. Viscosity indicates the time in seconds required for
the paint product to run through the hole in the cup. DIN4
measurement of viscosity is performed using a 1 dl cup;
at the bottom of the cup is a hole with a 4 mm diameter.
Measuring cups with a larger hole may also be used, e.g.
DIN6 or DIN8, where the diameters of the holes are 6 mm
and 8 mm respectively.
VOC (Volatile Organic Compound)
Volatile organic compounds; the VOC count indicates the
quantity of volatile organic compounds in grams per one
litre of paint (numerical value g/l).
Water-borne binder
A solid polymer (binder) that has been dispersed as tiny
particles in water.
Wetting lacquer (non-wetting lacquer)
Generally, lacquers are wetting. This property is due to the
binder they contain, which – as it wets natural-coloured
wood – darkens the colour slightly. Stained surfaces are
normally lacquered with wetting lacquer.
Non-wetting lacquer is used when it is desirable to retain
the natural lightness of light-coloured wood. Non-wetting
lacquer alters the colour shade of stains, and therefore it is
not used on top of them.
Whitening paste (White lacquering)
Often, a whitened birch or pinewood surface is desirable.
In that case, a white titanium-dioxide paste recommended
by the manufacturer can be added to the lacquer in order
to achieve a whiter result.
UV-protection
The UV-rays in sunlight cause yellowing in wood. Lacquers
that contain a UV-protective compound may reduce the
capacity of UV-radiation to penetrate the lacquer film, and
thus slow down and decrease such yellowing.
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17. Tinting systems
A
tinting system is the most economical way of
manufacturing a wide colour range of colour shades
based on customer needs. The tinting system consists
of base paint, colourants, a tinting machine, formula
management software and a mixer. A spectrophotometer
can be connected to the computer and formula
management system to allow the measurement of the
customer’s samples in order to generate a new colour
formula based on the colour sample.
Tikkurila Oy Industry uses four tinting systems, all
of which fall under the Temaspeed service concept.
Temaspeed is Tikkurila’s service concept for professionals
who use industrial paints. Through its pan-European
Temaspeed resale network, Tikkurila offers an extensive
selection of high-quality products, technical service
and product delivery. In order to cater to different types
of paint and different visual requirements in terms of
appearance, the following tinting systems are available:
Temaspeed Premium, Temaspeed Fonte, Temaspeed
Dicco Color and Temaspeed Akvi Tone.
Figure 41. Temaspeed HA 200 automatic tinting machine.
I N D U S T R I A L
The Temaspeed Premium is a solvent-borne tinting
system intended for tinting solvent-borne paints. The
Premium system includes 12 colourants and two base
paints – white TAL and the clear TCL. The number of
base paints has been reduced to two on account of the
excellent colour-saturation of the colourants. Having two
base paints helps dealers reduce costs and stock level.
The Premium system is used to tint Temadur,
Temacoat, Temalac, Diccoplast and Dicco Flex products,
among others. For special shades, special base paints
– containing, for example, larger or smaller aluminium
particles – are available. Aluminium base paints can be
used to make shimmering metallic colours – something
that cannot be accomplished with regular base paints
or colourants.
The Temaspeed Fonte tinting system is intended
for water-borne products. Fonte colourants are suitable
for tinting all industrial, water-borne products. These
colourants produce the best possible tinting result,
and also affect the product’s other properties, such as
drying time. The Fonte tinting system consists of 15
colourants that can be used to produce thousands of
different colour shades. Fonte colourants are used to tint
metal-industry products, such as Fontelac, Fontecoat
and Fontedur, and wood-industry products in the Akvi
and Pinja product series. There are also universal tinting
systems on the market; they are suitable for both solventand water-borne products. With using these systems,
some poor qualities, such as a stronger tendency of the
pigment to sink to the bottom, lesser colour saturation
and compatibility problems must be accepted.
The water-borne Temaspeed Dicco Color tinting
system has been developed exclusively for tinting wood
industry stains. Stains are required in many different
shades, and often in small quantities at a time. For this
reason, the most economical method of manufacturing
stain shades involves using a tinting system. In addition to
10 stain solutions, the system contains a colourless stain
solution, a tinting machine and formula management
software. The viscosity of the solvents is very low –
thereby differing from normal colourants and allowing
the stains to be absorbed into the wood evenly.
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55
The Temaspeed Akvi Tone tinting system has been
developed for tinting water-borne, transparent wood
stains, window lacquers and stains. The system includes
10 colourants, clear stain solution, a tinting machine and
formula management software. Due to their excellent
weather-resistance, exterior stain shades have a higher
resistance to sunlight than Dicco Color stains.
The Colour Management Software contains
tinting formulae for the most common domestic and
international industrial paint colour cards, such as RAL
and NCS. Shades are available in their thousands, and
their numbers are growing along with the new colour
cards and the latest trends. For special colour shades
and customer-specific colours new colour formulae can
be produced using a spectrophotometer and formula
management software.
Tinting is done using either a manual or an automatic
tinting machine. Weighing is also possible, but requires
considerably more time and increases the margin of
error. For tinting the colourant a hole is punched into the
lid of the base paint. In accordance with the formula for
the desired colour shade, the tinting machine dispenses
the correct colourant in the correct quantities into the
base paint. When the tinting is completed, the hole is
closed with a special plastic plug in order to prevent the
tinted paint from splashing out of the can when the can is
shaked. The can of base paint – to which the colourants
have been added – is shaken to mix the contents by
rotating or shaking it in a shaker. The end result is the
proper colour shade in ready-to-sell packaging.
Industrial manufacturing of small quantities of special
shades is very expensive. For this reason, there are
numerous tinting and distribution points in Finland and
around Europe. Tinting allows the customer to obtain the
desired batch of paint quickly and flexibly. The accuracy
of the colour shades is excellent, facilitating the purchase
of supplemental quantities without interrupting painting
work. Furthermore, the customer’s and the distributors
storage needs decrease, since it is unnecessary to keep
every possible colour shade in storage. Purchasing
small batches of paint becomes easier and quicker,
which also means considerable cost savings.
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T I K K U R I L A
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18. Solving finishing problems
F
inishing work ordinarily proceeds without incident. In some cases, however, problematic situations may be
encountered where a solution is not readily available. This section explains various surface defects and their
causes and provides guidance for solving them. The defect is explained under the heading. The cause is explained in
the left-hand column of the table, and the suggested solution in the right-hand column.
18.1 Orange-peel surface
The paint surface does not smooth out; instead it resembles the surface structure of an orange, i.e. it is covered with
a minor, slightly slanting unevenness.
Cause of the surface defect
Suggested solution
The wrong thinner has been used in the paint, or the amount of thinner is
insufficient.
Use the product-specific thinner mentioned in the product data sheet. Add
thinner to reduce operating viscosity and to improve evenness.
The difference in temperature of the surface and the paint is too great.
Level off the difference in temperature of the surface and the paint.
The pressure during spraying is too high, or the spray gun is at an improper
distance from the surface.
Correct the spraying pressure and distance.
The air circulation in the flash-off and drying oven is too high.
Lower the air circulation in the oven to allow the paint surface to even out.
The relative humidity of air is too low, and drying occurs too quickly.
Increase the relative humidity in the space or use a slower thinner.
Repair: Sand the uneven surface smooth and matt by using an intermediate sanding paper. Perform repair lacquering
as per the suggested solutions.
18.2 Poor coverage
The paint surface does not cover up the shade differences in the substrate, or the shade is wrong.
Cause of the surface defect
Suggested solution
The primer is visible – either too light or too dark – underneath the topcoat.
Use a tintable primer. Tint the primer with the topcoat.
Too much thinner has been added to the paint.
Make a new batch of paint using a smaller amount of thinner.
The pressure during spraying is too high, or the spray gun is at an
inappropriate distance from the surface.
Correct the spraying pressure and distance.
The paint has been poorly mixed, and the pigments are in the bottom of the
can.
Re-mix the paint or make a new batch of paint.
Repair: Sand the uneven surface smooth and matt by using an intermediate sanding paper. Perform repair lacquering
as per the suggested solutions.
18.3 Paint film breakage in the curtain coating
The paint film does not remain even during the curtain coating machine run; from time to time, breakages occur.
Cause of the surface defect
Suggested solution
The curtain head is too high.
Lower the curtain head if possible.
Air bubbles or foam form in the paint.
Lower the curtain head and place a drainage sheet in the drop chute to
decrease the height from which the paint drops down. Check to make sure
that the pump does not mix air with the paint.
The air circulation is too brisk near the curtain coater.
Close the painting facility doors or build a wind shelter at the point of the
curtain head.
Unwanted material has been mixed in with the paint.
Filter the paint batch, clean the curtain coating machine or take a new batch
of paint into use.
The paint viscosity is too low.
Thin paint will not retain an even film. Make a new, thicker batch of paint and
mix some of it with the old batch.
Repair: Sand down the failed surface until smooth by removing the entire paint film. Repaint using the curtain coating machine.
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18.4 Air bubbles during painting
Air bubbles appear on the wet paint surface during spraying or immediately thereafter.
Cause of the surface defect
Suggested solution
Air leakage either in the high-pressure spraying gun or the piping system.
Check the spraying equipment.
Open wood structure containing a lot of air, especially oak or ash.
The air in the wood appears on the paint film. Use slow thinning.
Pre-heat the wood surface prior to painting.
The spraying pressure or atomising air pressure is too high during spraying.
Decrease the spraying or atomising air pressure.
Wrong spraying technique – spraying is done from too close a distance to
the object; paint viscosity is too high; paint temperature is too low.
Spray further away from the object; thin and/or heat the paint.
A very recently mixed coating was used.
Prepare a new batch of paint in time for the air bubbles to have a chance to
disappear before painting.
Repair: Sand the uneven surface by sanding and perform the repairs. Repaint after repairing.
18.5 Air bubbles during the drying phase
The paint surface appears even after painting. As it dries, holes or air bubbles form on it.
Cause of the surface defect
Suggested solution
The temperature or air circulation in the evaporation zone is too high.
The surface on the paint film is drying too fast. Lower the temperature and
decrease the air circulation in the flash-off oven.
The evaporation time is too short.
The solvent has not been evaporated entirely from the paint surface before
drying. The evaporating solvent makes holes in the paint surface. Increase
the evaporation time or use a slower thinner.
Repair: Sand the uneven surface smooth and matt by using an intermediate sanding paper. Perform repair painting
as per the suggested solutions.
18.6 Ineffective drying and soft paint surface
The paint surface does not dry in the oven or the surface remains too soft.
Cause of the surface defect
Suggested solution
An unsuitable hardener has been added to the paint or the amount of
hardener is too small.
Prepare a new paint mixture.
Check the label or product data sheet for the correct hardener ratio. Use a
sufficient amount of the correct hardener.
The hardener may be outdated.
The hardener may have absorbed moist, especially in polyurethane
products. Use a new batch of hardener.
The painting line preheating temperature may be too low.
Preheating before painting makes the drying faster.
Check to see that the pre-heating is functioning well.
The temperature of the drying oven is too low.
Check the temperature of the drying oven.
Drying oven ventilation is ineffective.
Check the ventilation in the drying oven.
The drying time is too short considering the properties of the product.
Check the drying time for the product and adjust the drying-oven lead-time
according to the product properties.
The relative humidity of air is too high and air circulation is deficient.
Check the relative air humidity; a humidity level higher than 70 % slows
down the drying.
Elevate the drying temperature, since heat will dry the air.
Increase air circulation.
Make sure that the air filters in the oven are clean.
Decrease the amount of air re-circulated back into the oven.
Repair: Elevate the drying temperature and lengthen the drying time. Mechanically remove the soft paint surface and
repaint.
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18.7 Surface cracking (reticular)
The paint surface acquires a reticular surface structure during the drying process.
Cause of the surface defect
Suggested solution
The application amount is too large.
Reduce the application amount and decrease ventilation.
The shape of the object may cause too thick a paint layer to accumulate in a
certain area. Change the position of the object to be coated.
The amount of hardener is too high.
Evaporation is too fast and the drying time is too short.
Lengthen the flash-off and drying time.
Air humidity is too high.
Decrease the relative air humidity, e.g. by elevating the temperature.
Repair: Sand off the cracked surface and repaint after executing the suggested solutions.
18.8 Surface cracking (grain-aligned)
Grain- and veneer-aligned cracks appear on the lacquer surface.
Cause of the surface defect
Suggested solution
The painted object has been stored at too low a temperature or in too high
a humidity.
Check the storage conditions of the finished products.
Select a coating better suited to the conditions.
Humidity of the wood or veneer is too high before application.
Repair: Perform intermediate sanding of the cracked surface and then repaint it.
18.9 Sagging
The paint is sagging along the object’s vertical surfaces and the edges of flat parts.
Cause of the surface defect
Suggested solution
The paint layer is too thick.
Reduce the amount of paint to be applied.
Adjust the direction and distance of the automatic spray guns.
Make sure that all spraying guns are spreading the right amount of paint.
The viscosity is too low.
Use a thicker viscosity or a thixotropic product.
Repair: Remove the saggings through intermediate sanding and repaint the objects after implementing the suggested
solutions.
18.10 Colour differences in painting
When painting with the same batch of paint, colour differences appear on the product.
Cause of the surface defect
Suggested solution
The paint batch has not been mixed well before the finishing treatment.
Mix the paint batch well before beginning the finishing.
Viscosity varies in different batches of paint.
Always use the same viscosity throughout as was used when beginning the
work.
The application amount varies in different areas of the product.
Adjust the curtain head in curtain coating machine so that an equal amount
of paint is applied to all areas.
Adjust the spraying guns so that the same amount of paint is applied to all
areas. Check to ensure that the operating temperature of the paint is the
same throughout the process.
Priming has been done with different colours.
The topcoat is not thick enough to cover up the colour differences in the
primer.
Repair: Intermediate-sand the topcoat and repaint using checked colour and equipment.
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18.11 Colour differences in lacquering
The colouring of the wood surface is blotchy and clearly a different colour from the rest of the surface.
Cause of the surface defect
Suggested solution
Redness appears in pine when lacquered with an acid catalysed lacquer.
Too much time elapsed between the sanding of the wood and the coating.
Increase production lead time and decrease intermediate storage time
before finishing.
Too much hardener has been used. An over-dose colours light wood species.
The lacquer has come in contact with iron, for example while stored in a can
made of an unsuitable material.
The lacquer had become dyed in the can.
Water-borne lacquer has rusted the container and has been dyed brown.
Acid-curing lacquer has come in contact with iron, for example while stored
in a can made of an unsuitable material.
Repair: Remove the dyed lacquer layer and re-lacquer according to the suggested solutions.
18.12 Colour differences in staining
When staining with the same stain, the colour shade is blotchy, striped or otherwise uneven.
Cause of the surface defect
Suggested solution
A different amount of stain is applied to different areas during spraying.
Practice staining a new product and colour before staining a larger product
lot.
The sanding is either too fine or too rough.
Too fine a sanding may close the wood surface, preventing the stain from
being absorbed into the wood evenly. Too rough sanding leaves the wood
grains open, causing the wood to absorb stain differently in different areas.
The spraying is either too dry or too wet.
Too dry a spraying will dry before it has a chance to evaporate from the
wood surface, preventing the colorant from entering the wood. Too wet a
spraying leaves puddles on the wood surface, causing uneven absorption.
Stripes in staining.
The spraying gun is drawing. Clean or change the nozzle. Adjust the
atomising air to attain an even spraying pattern.
Repair: Sand off the stain from the surface and re-stain after repairing. Minor colour differences can be evened out by
adding a maximum of 15% of the stain used to the sealer and/or top lacquer. Water-borne stain is added to water-borne
lacquer and solvent-borne stain to solvent-borne lacquer.
18.13 Gloss variations
While using paint from the same batch, gloss varies in different parts of the object or between objects.
Cause of the surface defect
Suggested solution
The batch of paint was not mixed thoroughly.
Mix the batch of paint thoroughly before preparing the first batch to be used.
Doing so will ensure an even gloss throughout the batch.
Application amount varies.
Adjust the painting equipment so that an equal amount of paint is distributed
in all areas.
Check to ensure that the temperature of the paint remains standard when
used.
An outdated batch of paint was used.
If you are using a batch of paint mixed the day before (two-component
products), add a maximum of 30% of the old batch into the new one.
Repair: Intermediate-sand the earlier surface and repaint.
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18.14 Wrinkling (shrinkage linking)
The spread film contracts into a heap on top of the underlying film.
Cause of the surface defect
Suggested solution
The new coating is applied on top of the semi dried coatingt.
Lengthen the drying time of the preceding layer, increase the drying
efficiency by adding heat or increasing air circulation.
Enlarge the distance between objects while drying to allow the air circulation
to reach all the different parts.
The wrong thinner was used in priming or applying the topcoat.
Check the product data sheet for the right thinners for the products.
The thinner used in priming was too slow and did not have time to evaporate
completely from the paint film.
Unsuitable paint combination.
The base coat cannot withstand the strong solvents in the topcoat (for
example, alkyd + acid catalysed paint). Determine the functionality of the
treatment combination before using the coating.
Repair: Remove the failed paint film and repaint, starting with priming.
18.15 Mat blotches
The paint surface has blotches that are distinctly more mat compared to the entire surface.
Cause of the surface defect
Suggested solution
The coating contains air bubbles that do not have a chance to burst and
even out before drying.
Lengthen the flash-off time before placing the object in the oven. Use a
slower thinner.
Prevent the formation of air bubbles in the finishing equipment.
The substrate is porous and absorbs the coating unevenly.
Execute a more thorough priming prior to applying the topcoat.
The base coat or putty is porous and absorbs the topcoat unevenly.
Switch to a denser primer or putty.
Execute the priming more thoroughly.
Increase the amount of topcoat.
Repair: Do an intermediate sanding and repaint the surface according to the repair instructions.
18.16 Stripes
A distinct stripe can be seen covering the entire width of the paint surface.
Cause of the surface defect
Suggested solution
The nozzle of the spray gun is clogged or defective.
Clean the nozzle or replace with an intact one.
If the clogging is continuous, replace the nozzle with a larger one.
The automatic spray guns are installed incorrectly or the speed of the
feeding mat is incorrect.
Check the installation of the spraying guns and set the correct mat speed.
There are impurities or crevices in the curtain coating machine lips.
Clean the curtain coating machine lips or have them milled straight.
Impurities are mixed in with the paint; they are caught in between the curtain
coating machine heads.
Filter the paint or prepare a new batch of paint from a different production lot
Repair: Remove the striped paint surface and repaint after implementing the suggested solutions.
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18.17 Impurities and unwanted material on the paint surface
There is a disturbing amount of small-size impurities and foreign particles in the paint film.
Cause of the surface defect
Suggested solution
The impurities originate in the paint.
Filter the paint until clean from unwanted material.
The impurities originate in the equipment.
Dirt comes loose during use and is mixed in with the paint.
Remove the paint from the equipment and clean with thinner.
When cross-using water-borne and solvent-borne products, the paint that has
accumulated in the hose may come loose and form impurities in the paint.
The impurities appear during drying.
Unwanted material flies onto the paint surface in the evaporation or drying
oven.
Clean the ventilation filters in the oven.
Repair: Do an intermediate sanding of the trashy surface until rough and repaint according to the repair instructions.
18.18 Craters (Repulsion)
Crater-like areas appear on the paint surface; the paint escapes from them.
Cause of the surface defect
Suggested solution
There are remnants of silicone or oil on the object’s surface. Inside the
factory, release or polishing agents or lubricants have been used, containing
silicone or oil.
The object’s surface becomes oily due to compressed air.
Discontinue the use of the substance containing silicone or oil.
Find out about objects outside of the factory onto which silicone may have
been applied.
Check to ensure that the compressed air filters are clean.
The thinner is too fast.
The paint surface does not have time to settle prior to drying. Switch to a
slower thinner.
Repair: After reviewing the suggested repair solutions, remove the paint surface containing craters and re-apply the
treatment. In some cases, the silicone may have been absorbed into the wood, resulting in a re-occurrence of the
repulsion.
18.19 Poor adhesion
The topcoat adhesion to the primer is insufficient.
Cause of the surface defect
Suggested solution
Poor sanding in between paint layers.
Sand more thoroughly and more strongly prior to applying the topcoat.
Too long a time elapsed between intermediate sanding and applying the
topcoat.
The base coat had a chance to harden too much before the topcoat was
applied.
The topcoat is sprayed when too dry; as a result, it does not possess the
capacity to adhere adequately to the base coat. The topcoat is too quick to
dry.
Increase the amount applied.
Adjust the spraying gun controls so that the result is a wet topcoat.
Adding too much hardener to the paint.
Check the mixing ratio for the topcoat before finishing.
Too much of the old paint mixture has been used. The paint portion has
already had a chance to react partly with the hardener, resulting in a
weakened adhesive capacity.
Use only the new paint mixture or add a maximum of 30 % of the old batch
into the new one.
The wrong base and topcoat combination was used.
Check the compatibility of the paint mixture ingredients from the product
data sheet or from the finishing combination.
Repair: After reviewing the suggested repair solutions, remove the poorly adhered layer of paint and repaint according
to instructions.
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I N D U S T R Y
18.20 Greasiness (Sweating)
The finished surface is blotchy, feels greasy and smells sour.
Cause of the surface defect
Suggested solution
Adding too much hardener for acid catalysed paint.
Always check the product data sheet for the correct mixing ratio before
finishing.
Repair: Remove all of the seating paint film from the entire surface and repaint.
18.21 Light blotches on the lacquer surface
The lacquer surface is not completely clear after drying; instead, milky white spots appear here and there.
Cause of the surface defect
Suggested solution
In cellulose-based products, air humidity has condensed on the lacquer film
during spraying.
Prevent humidity from penetrating the lacquer surface. Dry the air at the
spraying point.
Switch to a less humidity-sensitive lacquer product.
A thinner incompatible with the lacquer was used.
Check the product data sheet for the correct thinner.
Use a more slowly evaporating thinner.
The spraying surface of water-borne products is uneven, causing water to
remain in some areas of the surface after drying.
Check to ensure that an even, cohesive lacquer surface is achieved on the
object’s entire surface during spraying.
In water-borne, UV curing products, evaporation prior to the UV-lamps is
insufficient, and water remains inside the lacquer film.
Repair: Sand off the milky surface in its entirety and re-lacquer after executing the suggested repairs.
18.22 Rough surface
The paint surface is uneven after drying. It also feels rough to the touch.
Cause of the surface defect
Suggested solution
The environment and finishing equipment are dirty or dusty.
Clean up the surrounding environment and the equipment used.
The paint mixture was not mixed sufficiently; as a result, more solid
ingredients may remain at the bottom of the container.
Mix the paint thoroughly before use.
The spraying gun pressure or nozzle or the spraying distance is incorrect.
Set the correct values for spraying. Avoid setting the pressure too high and
optimize the nozzle size and spraying distance.
The reverse side of the objects may become rough due to the impact of
overspray.
The paint dries too quickly relative to the spraying values set.
Repair: Intermediate-sand the rough surface until smooth and re-coat after implementing the suggested solutions.
I N D U S T R I A L
W O O D
F I N I S H I N G
63
19. Colours
19.1. Colour is light
The world we live in is full of colour. Colour provides
a means for brightening up our environment. Colours
and the way we use them in decorating directly affect
our moods and feelings. Compatible colours create a
harmonious balance and put us in a good mood.
Light is a pre-requisite for colour – in darkness we do
not see colours. The sun radiates light and is the primary
source of it. However, most objects in our environment do
not radiate light. They are what are known as secondary
sources of light. We notice them and their colours only
when light. Seeing light requires three things: light,
object and a viewer. The colour shade and object we
see are formed as a result of their synergy.
Crest of a wave
Hollow of a wave
Waves are classified either according to length or
the number of vibrations per second. Wavelength is
defined in kilometres, metres, centimetres, millimetres,
nanometres or picometres. The number of vibrations per
second – i.e. their frequency – is measured in hertz, Hz.
1019
Wavelength
Viewer
Frequency (Hz)
Light
Figure 44. A light wave.
0.1 Å
Gamma rays
{10.1Å nm
1018
X-rays
1 nm
1017
400 nm
10 nm
1016
Ultraviolet
100 nm
1015
Visible light
Short-wawe infrared
10
14
Infrared
{
1000 nm
1 µm
500 nm
600 nm
10 µm
1013
Termal
infrared
100 µm
1012
Long-wawe
infrared
1000 MHz
1011
UHF
Figure 43. Seeing colour.
Object
Micro wawes
1010
500 MHz
{
1000 µm
1 mm
1 cm
Radar
10 cm
109
VHF
7-13
Light consists of radiation that travels at an
exceedingly high speed – approximately 300,000
kilometres per second. More precisely, light consists
of electromagnetic vibration, which ripples out from its
source in a wave-like fashion. Like a water wave, each
light wave has a crest – and a hollow.
100 MHz
1m
108
Radio, TV
FM
VHF
2-6
50 MHz
10 m
107
100 m
106
AM
1000 m
Long wawes
Figure 45. An electromagnetic spectrum.
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T I K K U R I L A
O Y
I N D U S T R Y
700 nm
Waves of different length have different properties.
X-rays, for example are utilized in making medical
diagnoses, while many a home has a microwave oven.
Other types of waves are used to transmit both radio and
television programmes.
Only a minuscule quantity of electromagnetic
radiation is visible as coloured light. The spectrum of
light visible to the eye is between 380 nm (ultraviolet
light) and 780 nm (infrared light). With a prism, light can
be split into its colour components. White light, which
consists of the colours of the spectrum, can be split into
all the colours of the rainbow.
• Some of the light is absorbed, the rest passes through
the object: the hue of the colour we see depends on
which wavelengths are absorbed and which pierce the
object.
• Some of the light is reflected and the rest passes
through the object: under these conditions, the amount
of light that is reflected and the amount of light that
passes through the object are changed.
The properties of the light object determine which of
the above-mentioned events occur.
Red (approximately 700 nm)
Figure 47. One of the properties of objects is their capacity to
absorb or reflect the colour that possesses a certain wavelength.
Green (approximately 550 nm)
Blue (approximately 400 nm)
Figure 46. Wavelengths shorten as the colour changes from red to
green and blue.
19.2. Colour made visible
Colour cannot be regarded as a characteristic of an
object, like its shape. However, objects do have the
characteristic of absorbing or reflecting colour that
possesses a certain wavelength. When white light
reaches an object, one of the following occurs:
• All of the light is absorbed: the object appears black.
• All of the light is reflected: the object appears white.
• All of the light passes through the object: the colour of
the light does not change.
• Some of the light is absorbed, the rest is reflected: the
hue of the colour we see depends on which wavelengths
are reflected and which are absorbed.
I N D U S T R I A L
We notice only those colours which correspond to
the reflected wavelength. If the object reflects – of all
the colours in the spectrum – only the wavelength of the
colour red, the object appears red, and a person will
see it as red. A white object reflects the whole spectrum,
and consequently, the object will appear white. A black
object absorbs all of the light into itself, and will therefore
appear black.
The human eye receives the light that the object
reflects or the light that has passed through the object.
The lens focuses the image on the light-sensitive retina,
transforming it into nerve impulses, which in turn evoke
the perception of colour in the brain. The sensation of
sight, the perception of colour, arises when the brain
interprets the perception created on the retina.
painted surface
Figure 48. Colour made visible.
W O O D
F I N I S H I N G
65
The retina inside the eye contains two types of cells –
rods and cones – each of which specializes in detecting
light of different strengths. Rod cells detect details in
very dim lighting and recognize the difference between
brightness and darkness. Cone cells are shorter than
rods, and it is cones that enable us to distinguish
colours. The relative majority of cone cells are located
in the fovea – they can also be found in much sparser
concentrations in the border areas of the retina. There are
three types of cone cells that are sensitive to the different
wavelengths of light. Some of them react to light between
400-500 nm, making them sensitive to blue light. Other
cone cells “see” the light between 500-600 nm, i.e. only
green light. The third type of cone cell is sensitive to red
light, which occurs between 600-700 nm. This collection
of rods and cones makes the human eye sensitive to
such a degree that it is able to receive and distinguish
millions of colours.
In the early 19th century, the scientist Thomas
Young stated that the human eye perceives light as a
mixture of three main colours (red, green and blue).
This characteristic is utilized, for example, in computer
screens. The computer can be used to establish what
is known as the RGB value (R = red, G = green and
B = blue) for all colour hues. The RGB values are based
on additive colour formation.
Figure 49. Additive colour formation.
Figure 50. Subtractive colour formation.
19.3. Colour formation
19.3.1 Additive colour formation
Additive colour formation means placing different colour
lights on top of one another. The principle of additive
colour formation is utilized in colour TV, computer screens
and theatres when all colours of the visible spectrum are
produced. The principle can easily be explained with
three slide projectors, each of which forms one additive
basic colour on the screen. The three main colours of
light are red, green and blue. A mixture of the main
colours forms the colour white. The new colour areas
created are of a lighter hue than their components.
19.3.2 Subtractive colour formation
Subtractive colour formation is discussed, for example,
in connection with tinting paints. If colorants – such as
paints – are combined instead of lights, the mixed colour
areas are always darker than their components. Some
of the white is removed in order to attain the desired colour. The main colours include blue, red and yellow. A
mixture of the main colours is black in theory, but brown
in actuality.
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T I K K U R I L A
19.4. Defining and measuring colour
Colour shades can be assessed with the eye or with
a measurement device. Among the advantages of
defining colour with a device are recording the colours
measured, repeatability and secure functioning; the
background and lighting do not influence the measured
result. Therefore, it is best to use a spectrophotometer
to check the colour shade. The spectrophotometer is
based on the three-colour system. It measures the red,
green and blue measurement values of each object and
communicates them via a computer. With the measured
values, the colour can be replicated on a computer
screen, for example.
The colours found in nature are never pure colour
shades – they contain all the sub-areas of the visible
wavelength. The colour of an object depends on which
wavelength area it reflects the most. A more exact
measured result of the colours in nature is obtained
with a spectrophotometer – a numerical measured
result, i.e. a reflective graph is obtained for each colour
shade, allowing the different shades to be distinguished
O Y
I N D U S T R Y
from one another. The spectrophotometer’s sensor
measures the value of the colour shade within several
wavelength areas. A reflective graph can be run of the
measured result; a computer can be used to convert it
into numerical L*a*b values (CIELab, the colour space
definition of the International Commission on Illumination
(Comission Internationale de l’Eclairage). In this kind
of colour space, the image of a colour impression is
presented using three components: hue, saturation and
brightness.
The measurement values of the spectrophotometer
are indicated with the letters L, a, and b – where L stands
for the degree of lightness-darkness, a stands for the
red-green axis, and b for the yellow-blue axis. With these
numerals each colour hue can be defined in the colour
space, which is often depicted as a colour ball. The hue
changes when circling the ball – saturation increases the
further outward you go from the interior; white is located
at the “north pole” of the ball, while black is located at
the “south pole”.
19.6. Metamerism
Light always influences the colour we see. This change of
colour in different lighting is called metamerism – it may
seem that an object changes colour when the source of
lighting changes. Metamerism is observed under three
different types of standard lighting conditions – daylight,
incandescent light and fluorescent light. When shades
are manufactured according to colour models, an effort
is made to make the new colour appear the same in all
of the above-mentioned lighting conditions. If a colour
model is highly metameric, the shade is synchronized
only in daylight or in the lighting conditions in which the
product is most often used.
19.5. Differences in colour
In colour space, colour differences are calculated as the
hue value distance and are indicated with a dE marking.
The dE value is not, however, an absolute value – its
correctness depends on the colour area measured. In
light and white hues the dE value must be under 0.5 in
order for the hue to look the same. In the red colour area
a colour difference corresponding to a dE value as high
as 3.0 may appear to be the same. An effort is underway
to improve the accuracy of describing colour hues.
More recent modelling methods, such as CMC2:1 and
CIE2000 are also in use.
I N D U S T R I A L
W O O D
F I N I S H I N G
67
Sources
Helsinki Technical School (Heltech),
Käpylä education unit, Chromatics teaching materials, 2005
Internet: http//www.puuinfo.fi/, http://www.puuproffa.fi/
Mirka Oy: Tehokasta puunhiontaa (Effective wood sanding), 2005.
Photographs:
Kährs
Penope Oy
Tikkurila archives
68
T I K K U R I L A
O Y
I N D U S T R Y
High quality and ecoefficiency of our products
are Tikkurila’s first priority
Since 1991, the high quality of Tikkurila’s products and
services has been guaranteed by a quality management
system in accordance with the ISO 9001 standard. The
components of high quality are competent personnel, strong
product development, advanced automation of production
technology, and fast and reliable delivery.
At Tikkurila, environmental, health and safety issues have
been brought together under the same management system
as issues pertaining to quality – operational development is
critically important and the goal is continuous improvement.
Tikkurila’s operational methods meet the requirements of the
International environmental management standard ISO 4001,
and the Occupational Health and Safety Standard OHSAS
REG.NO. FI - 000001
18001, as well as the conditions of the Eco-Management
and Audit Scheme (EMAS) of the European Union.
In addition, the company follows the social responsibility
principles laid out in the “Tikkurila, the Environment and
Community Programme”. Our aim is to enhance the wellbeing of all of our stakeholders and the well-being of the
environment throughout our operations.
Tikkurila’s own environmental programme is based on the
chemical industry’s international environmental, health and
safety programme entitled Responsible Care.
.
Tikkurila Oy
P.O. Box 53, FI-01301 Vantaa, Finland
Telephone +358 9 857 71
info.coatings@tikkurila.com
www.tikkurila.com