How to choose paints and varnishes for wood for external work

Recommendation points

• General classification
• What properties should coatings have?
• Difference between paint, glaze and varnish
• Dilution and mixing rules
• Methods for applying varnishes and paints

Paints and varnishes for outdoor use do not differ in environmental safety, but at the same time provide high-quality protection of wood. Today we will deal with the varieties of such coatings, their area of ​​application and the rules for applying to wooden surfaces..

General classification

The paints and varnishes we are considering today are designated by special terminology as “Protective and decorative weather-resistant means for wood processing”. This name reflects the main functions, the set of which characterizes a separately taken composition and sets the scope of its application. So, according to the first criterion, they determine whether the coating is capable of carrying a decorative function, or is it intended solely to protect the tree. The second sign is at what stage of processing this or that tool is used: for example, in a three-stage opening, impregnation is applied first, then primer, and last of all – enamel or varnish.

Such a high variety of properties would be impossible without varying the chemical composition. Mainly the properties of the coating depend on the binder, which can be film-forming or not, based on alkyd and polyurethane resins, or on an aqueous dispersion or other base. Finally, additives play an important role: polymerization accelerators, UV filter formers and others like them. In the latter aspect, the difference will be noticeable only for paintwork materials from different manufacturers, that is, two PF-115 enamels of different brands will both be based on pentaphthalef resins, while the content of desiccant, solvent and pigments in them will be different. This, in turn, affects the quality and cost of paint..

The third sign by which materials differ is decorative properties. Wood is a material with a high aesthetic value, respectively, and paints can either cover the surface with a monochrome opaque film of varying degrees of gloss, or tint the wood itself to a certain depth. Paints with different hiding power change the texture of wood in their own way. Thus, stains and glyphtal enamels can be used to treat brushed wood, which gives a very interesting visual effect. The same applies to varnishes: they can tint wood with different intensities, highlight texture or contain a fraction of the usual coloring pigment..

What properties should coatings have?

Most industrial wood species require protective treatment. The properties that coatings impart to a wooden product are determined by the wood’s vulnerability to the following factors:

1. Humidity – when swelling and drying, lumber changes their dimensions linearly and orthogonally. Film-forming paints and varnishes reliably protect wood from getting wet, reducing absorbency to almost zero.
2. Pathogenic microflora– one of the main reasons for rapid aging and loss of strength. The ability of a tree to be a breeding ground for microorganisms is eliminated in two ways: by treatment with antiseptics that kill mold, and also by preserving the tree with the help of film-forming coatings.
3. Insect pests– the factor is similar to the previous one, the methods of counteraction are the same. The only difference is that instead of an antiseptic, insecticides are used, and preservation includes, among other things, the blockage of large vessels, due to which soft fibers are cured.
4. Flame – wood has a G4 flammability group, it can be reduced by proper processing. Impregnation with fire retardants and the inclusion of the latter in the finishing coat allows to provide a group of flammability of wood products up to G2-G3, which expands the scope of their application in construction and exterior decoration.
5. sunlight– on it, the wood turns gray within a few years. Transparent coatings include a photo filter that traps ultraviolet radiation and does not allow it to have a harmful effect on the surface of the tree.
6. Temperature fluctuations. The general protective ability of the paint is determined by the chemical composition of the base: whether it regains its shape after heating and whether it retains plasticity in frost. In this regard, polyurethane and alkyd enamels show themselves better than others, and oil enamels are the worst..
7. Pollution – due to its soiling and lack of antistatic properties, wood is a fairly easily soiled material. However, the presence of a film on the surface and clogged pores make it possible to remove dirt quickly and easily, while the wood becomes less susceptible to aggressive chemicals..

It is not always necessary that all the listed properties are combined in one paint or varnish. So, the details of a wooden grillage can simply be protected from moisture, insects and organics, but for natural siding you will need a composition that is resistant to fading, is well washed and restricts combustion. Basic and special properties are usually indicated by symbols on the label.

Difference between paint, glaze and varnish

Let us dwell in more detail on the decorating properties of paintwork materials on wood. There are three basic ways to cover wood: preserving texture, texture and both. Depending on the goal, various formulations and methods of their application are used. And if work with enamel rarely differs in specificity in relation to wood, then glazing and varnishing compositions require a more selective approach.

With varnish, everything is most simple: it belongs to film-forming paintwork materials, that is, it creates an impenetrable shell on the surface of the tree. The varnish can be transparent, tinted in color from pale amber to chocolate, as well as tinting, that is, containing pigments in an amount of 5-10% of the total mass. Water-based varnishes are extremely rare, mainly based on resins based on organic solvents. Thus, depending on the base of the varnish, its different absorbency can be recorded not only for different types of wood, but also for fibers of different density in one product. These differences are most clearly visible when covering with tinted varnish: the emphasis on texture is clearly observable, therefore, the right decision when choosing a transparent coating will be to have a sample of the wood from which the workpiece is made at hand for applying samples.

Lapis lazuli (glaze) is a kind of intermediate link between paints and varnishes. The concept of glazing paints encompasses two rather large groups: tinted translucent, as well as completely opaque and non-film-forming upon drying. Thus, in the first case, the visual texture is partially preserved, and in the second – the roughness of the wood and its relief pattern. Note that color and richness of texture do not play a decisive role for translucent glazes. At the same time, compositions of the second type require careful processing of parts, because this kind of coating is not able to hide surface defects.

Dilution and mixing rules

Paints for outdoor use are subject to less stringent sanitary and environmental requirements. Water-soluble dispersions for external woodworking are rare, except in the range of facade paints, for which application to wood is simply allowed. The vast majority of paintwork materials for wood for external use are based on resins diluted in an organic solvent or polymerized oils (drying oil).

It is possible to mix paints, although it does not make sense. Basically, this is done when processing parts that have no decorative value. However, you should be careful: only oil-based paints are fully compatible with each other. When mixing them, the loss of color saturation is the least noticeable, but the mixing process itself must be very careful. Water-based paints can only be mixed with each other, but not always. For example, latex paints tend to lose their characteristic surface texture. In some cases, stratification or precipitation of the dispersion is possible. Compatibility of paints on the same type of binder is guaranteed. The main disadvantage of such mixing is the irreversible loss of saturation of the final color, therefore it is better to mix paints before tinting..

Sometimes the consistency of the paint has to be adjusted depending on the application method. If you need a more liquid paint, use the same solvents that are included in its base. Sometimes it is necessary to accelerate the hardening by adding desiccants, for example, to apply a thicker layer in one pass with a spray gun, requiring the paint to cure faster. Desiccants should be used more selectively: it is better to use those that are included in the composition, which is quite easy to determine according to GOST for paint. But there are also universal driers for alkyd and oil-based paints, which have additional properties, for example, delayed action or reduced spreadability. Hardening accelerators are almost never used when working with water-dispersed paints, because they already dry very quickly.

Methods for applying varnishes and paints

The paint you choose must be suitable for the tool you have chosen, as indicated by the corresponding pictograms on the label. But that’s not all: the paint must have a certain consistency, which is usually achieved by mixing it with a significant amount of solvent. Hence the exaggeration about the horrendous consumption of spray guns: in their tanks sometimes there is up to 50-70% of the solvent, which in itself is quite cheap. Most atomizers come with a Shell or Ford funnel for measuring viscosity, as well as instructions and optimal parameters for this tool..

Sprayintended for painting embossed products and wide areas where minimal color difference is required. The most useful spray gun (even a manual electric one) is when processing carved parts, especially with an abundance of small through holes. The smaller the size of the parts to be painted, the narrower the torch must be, otherwise the paint will scatter uselessly.

Roller painting– Ideal for large surfaces, mainly due to its high application speed. The roller applies to the surface exactly as much as can remain on it, respectively, and the uniformity of the coating is higher. A roller is useless with an abundance of embossed elements, painting siding with it is a complete torment. The other two ways are – swab and brush– used where none of the previously listed ones came up. Basically, these are hard-to-reach places where the roller did not reach, as well as complex-shaped parts, such as the walls of a log house.