The durability and appearance of the finished surface of an object depends not only on the coating, but also on the choice of basis and coating technology which are extremely important factors.
The base has to be prepared thoroughly. Cavities and scratches have to be filled up. Apart from sanding before the coating application, it is necessary to repeat the sanding procedure after each applied layer of coating. The following points should be considered when choosing a basis:
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Every kind of wood is sensitive to scratches and impacts.
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It is very difficult to provide beech wood with a substantial resistance to grease since the large pores allow grease to penetrate under the coating easily.
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Wood with large pores has to be treated with a primer with good wetting properties since liquids can later penetrate the wood under the coating.
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Dirt and dust are more conspicuous on dark and glossy surfaces.
Less resistance on the finished wooden surface
compared to earlier tests might have the
following reasons:
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Application temperature too low, insufficient ventilation or increased air humidity in the working area.
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Insufficient coating thickness.
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Stacking and packing too soon causing objects not completely dry to stick to each other. Also solvents not completely vaporised affect the adhesion between the basis and the coating.
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The coating material was not properly stirred before application.
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Improper dosage of hardener.
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The use of old coating and hardener.
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Microscopic air bubbles in the coating caused by a low application temperature, too high a viscosity and an incorrect choice of thinner.
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Unsuitable basis and coating.
QUALITY
OF
COATED
WOOD
Wood is generally a soft and porous material that easily attracts dirt and moisture.
Coating is used to improve the resistance of the wooden surface against mechanical and chemical impacts, to assure biological protection (when used outdoors) and to make the wood surface attractive.
GLOSS
Gloss is one of the aesthetic properties of a coating. In accordance with measurements achieved by the application of the Gardner method (60°), the finished surface is classified as follows:
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Matte: Gloss less than 10
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Half-matte: Gloss 10-35
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Half-gloss: Gloss 35-60
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Gloss: Gloss 60-80
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High-gloss: Gloss more than 80
OPAQUE AND TRANSPARENT COATINGS
Opaque as well as transparent coating systems can be applied to a wooden surface. A uniform colour is provided by a opaque system, which consists of a primer and a top coat. The natural beauty of the wood and its structure are emphasised by a transparent system, which consists of a primer and a top lacquer. Pigments give wood a specific colour, they imitate high-grade wood kinds, increase the resistance of the light-sensitive wooden surface against UV-radiation, they hide biological damage to the surface and achieve other special effects.
PORES
Depending on the coating, which either completely fills up the pores or only covers their inner walls, the coating giving filled or open pores can be applied onto the wooden surface.
DURABILITY
The durability of the surface determines the ability of the finished surface to resist scratches, impacts, wear and tear, thermal changes and controls the swelling or shrinking of the wood. The chemical durability determines the resistance of the finished surface against liquids such as coffee, water etc. Different kinds of tests are done to determine the durability of the wood surfaces against chemical and mechanical impacts. Some of the most common methods are:
EN 12720 - Evaluation of surface durability against cold liquids (water, coffee, alcohol)
EN 12721 - Evaluation of surface durability against damp heat
EN 12722 - Evaluation of surface durability against dry heat
SS 839117 - Evaluation of surface durability against scratching
ISO 4211-4 - Evaluation of surface durability against impact
EN 438-1:2 - Evaluation of surface durability against abrasion
SS 839120 - Evaluation of durability of front edges against water
Depending on the final purpose of the finished objects, certain demands on the finished surface are raised regarding appearance, chemical and mechanical resistance.
The importance of base and coating technology
USEFUL ADVICE
In this section a number of important recommendations regarding the application are given.
Following these recommendations will lead to a good end result, to less problems and work stops, to optimise the productivity and to decrease the production costs.
It is important to read the guidelines for the applied materials as well as the safety instructions.
It is necessary to track down stock movements. Use the FIFO-method (first in first out); which means that the material leaves the stock in the order that they came in.
The finishing system (base, coating, sanding etc) has to be pre-tested regarding their properties for coating, storage, transportation and usage. If any property has been amended it is advisable to retest the whole system.
Check the coating equipment daily. The coating area must be kept clean and orderly in order to assure work safety, fire safety and the coating quality.
Make sure before starting the application that the paint material has the required colour and gloss.
The basis has to be sanded before the coating application. The sanding has to match the technological demands since it has a serious impact on the coating quality and consumption.
The treated objects must be dust-free before they enter the application area. This will reduce the dust level inside the application area greatly.
Make sure that the basis suits the required coating type. It is possible that the basis properties (veneer, chipboard, fibreboard etc) do not suit the chosen coating system.
Repairing defects and damages to the surface during the coating process is time-consuming and expensive. A special area for repair work should be determined.
The surface must be clean. Oil, wax and silicone can cause non-uniform drying, spots and craters.
In order to simplify the handling, the coating material should be mixed in their original containers. Two-component coatings (especially acid-cured coatings) should be stored after mixing in stainless steel or plastic containers since the hardener might otherwise react on the metal of the container and discolour the coating pink.
Add the precise quantity of hardener as per the instructions. Use graded litre measures when mixing the coating material. Using an approximate dosage of the hardener or dosage by using a stick does not provide the necessary precision. Too much hardener makes the coating brittle, and too little hardener may result in a slower drying time.
Use only the thinner indicated in the instructions. The choice of the thinner depends both on the coating and the application.
The coating has to be stirred thoroughly before application as pigment and other additives may form layers during storage. Insufficient stirring can cause differences in gloss and colour. After stirring, the coating needs some time for the elimination of air bubbles.
The viscosity of the coating should be chosen in accordance with the application method, so it is very important to check and adjust the viscosity. This should be done at a temperature of +20°C since the required amount of thinner depends on the temperature. When applying the coating by a curtain coater or a roller coating unit, the viscosity should be checked at the beginning of the work, after each break, after re-supply and once every hour during the treatment process.
Special notice should be taken regarding the pot life of the coating because the curing process is initiated immediately after adding the hardener. This may take - depending on the type of coating – from a couple of hours to several days. Coating residues with a long pot life can usually be used the following day. In this case it is recommended to combine the new and the old
mixed coating material with a mixing ratio of 2:1. It is advisable that the residues which will be used the following day are stored in a cool place. Use clean containers to store the residues of the coating. Seal the containers and mark them with date, product code, and name.
The thickness of the applied coating depends on the quality of the wooden surface and on the requirements on the appearance - with open or closed pores. The material consumption shown in the instructions may vary in reality. Too thick a coating may result in cracking, too thin may affect the appearance and the durability against chemical and mechanical impacts.
The coating is usually applied to the surface in several layers which makes the adhesion between them extremely important. Intermediate sanding is one of the most significant conditions for good adhesion. This is especially important for acid-cured, waterborne, UV-cured and polyurethane materials where the top layer doesn’t absorb the previous layer as with nitrocellulose materials.
To improve the adhesion of applied UV-materials it is possible to partially cure the bottom layer to the gel substance and to finalize the curing process after each layer.
Both the intermediate sanding and the application of the next layer should be done after the final curing of the previous layer. The quality of the sanding belt and the sanding speed should be checked during the process of sanding.
Too low a sanding speed does not give the required sanding quality, whilst a speed which is too high may result in polishing and basis overheating.
Solvents and coating conditions are also very important for the adhesion. If the thinner used is too fast and the air humidity too low the thinner evaporates too quickly whereby no substantial adhesion with the previous layer is achieved.
Thinners suitable for all properties are recommended in the technical data sheet, or by a technician. Unknown thinners and thinners not recommended by the coating supplier should be avoided.
The viscosity of the coating and the surface temperature has to be checked at the start of the coating application. The characteristics can be negatively affected if the surface of the coating has a temperature which is too low (recommended temperature is +20°C).
At the end of the coating process, but prior to stacking, the objects have to be cooled down to a maximum of +35°C.
The product technical data sheets and Safety Data Sheet should always be carefully reviewed, especially when using a product for the first time.
1. Check to make sure that the substrate is clean
Oil-, wax- and silicone-based impurities cause wetting problems or glossy blotches and hinder drying.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
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Mix the opened can before adding the hardener or thinner. Mixing the paint/lacquer is particularly important when using thixotropic products. By mixing, “false thixotropic” is removed, and excessive dilution of the product is avoided.
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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.
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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.
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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.
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.
18. 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.
19. Solvent
A component in solvent-borne paints, whose purpose is to dissolve the solid binders (resins) and polymers, and to reduce their viscosity.
20. Solvent-free paints
Paints that do not contain solvents; for example solvent free epoxies and 100 % UV products.
21. 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.
22. 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.
23. 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, micrometers 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.
24. 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.
25. Thixotropic
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.
26. Tinting system
A precise, quick and economical manufacturing method of colored paints suitable for products for which basic paints exist. The tinting system consists of tinting pastes, base paints, tinting formula files, a tinting machine and mixer (shaker).
27. UV-drying
Paints and lacquers that are specially developed for UV-drying are dried with UV-lamp radiation. These products attain the required chemical and mechanical properties only subsequent to the UV radiation.
28. 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.
29. 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.
30. VOC (Volatile Organic Compound)
Volatile organic compounds; the VOC count indicates the quantity of volatile organic compounds in grams per one liter of paint (numerical value g/l).
31. Water-borne binder
A solid polymer (binder) that has been dispersed as tiny particles in water.
32. Wetting lacquer (non-wetting lacquer)
Generally, lacquers are wetting. This property is due to the binder they contain, which – as it wets natural-colored wood – darkens the color 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-colored wood. Non-wetting lacquer alters the color shade of stains, and therefore it is not used on top of them.
33. 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.
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
1. Acid catalyzed 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.
2. Acrylate, acrylate polymer
Acrylate is a binder that is commonly used in waterborne products, among others.
3. 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.
4. Density
Density indicates the weight of one liter of paint at 23°C. The unit of density is kg/l as defined according to the standard SFS 3635.
5. 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.
6. Dry-film thickness
The thickness of the coating remaining on the substrate (μm) when the solvents have evaporated and the film has hardened.
7. Evaporation time
Please see Flash-off time below.
8. 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.
9. 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 flash off time, the paint surface may boil while in the drying zone.
10. 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 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 Nominal marking
90 ≤ x Full gloss
80 ≤ x < 90 High gloss
60 ≤ x < 80 Gloss
35 ≤ x < 60 Semi-gloss
10 ≤ x < 35 Semi matt
5 ≤ x < 10 Matt
< 5 Full matt
11. 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.
12. 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.
13. Nitrocellulose
Cellulose polymer (binder) is made from cotton and the cellulose in wood. Lacquers are made by dissolving nitrocellulose in solvents. Another name for nitrocellulose is cellulose nitrate.
14. 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.
15. Operating viscosity
Operating viscosity indicates the paint products viscosity after the hardener and the thinner has been added. The paint mixture must have the right operating viscosity in order to avoid possible defects.
16. 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).
17. 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. 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
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Add the right amount of thinner and mix in well with the paint mixture. The mixing time may now be cut in half.
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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.
7. Add the right amount of the right hardener
It is important to follow the recommendations for two component 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 catalyzed products, excess hardener will usually rise to the surface as mildew.
8. When using acid catalyzed products, mix the coating in a separate container
The two-component coating, which contains the hardener, may turn a reddish-brown color 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.
9. 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.
10. 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 disturbancesin drying, or variances in shade or gloss.
11. 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.
12. 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.