TYPES OF COATINGS
Upon completing this section, you should be able to identify the types of structural coatings and finishes, and the general characteristics of each.
As a Builder, you must consider many factors when selecting a coating for a particular job. One important factor is the type of coating, which depends on the composition and properties of the ingredients. Paint is composed of various ingredients, such as pigment, nonvolatile vehicle, or binder, and solvent, or thinner. Other coatings may contain only a single ingredient.
In this section, well cover the basic components of paintpigment, vehicles, and solventsand explain the characteristics of different types of paint.
Paint is composed of two basic ingredients: pigment and a vehicle. A thinner may be added to change the application characteristics of the liquid.
PIGMENT. Pigments are insoluble solids, ground finely enough to remain suspended in the vehicle for a considerable time after thorough stirring or shaking. Opaque pigments give the paint its hiding, or covering, capacity and contribute other properties (white lead, zinc oxide, and titanium dioxide are examples). Color pigments give the paint its color. These may be inorganic, such as chrome green, chrome yellow, and iron oxide, or organic, such as toluidine red and phthalocyanine blue. Transparent or extender pigments contribute bulk and also control the application properties, durability, and resistance to abrasion of the coating. There are other special-purpose pigments, such as those enabling paint to resist heat, control corrosion, or reflect light.
VEHICLES, OR BINDERS. The vehicle, or binder, of paint is the material holding the pigment together and causing paint to adhere to a surface. In general, paint durability is determined by the resistance of the binder to the exposure conditions. Linseed oil, once the most common binder, has been replaced, mainly by the synthetic alkyd resins. These result from the reaction of glycerol phthalate and an oil and may be made with almost any property desired. Other synthetic resins, used either by themselves or mixed with oil, include phenolic resin, vinyl, epoxy, urethane, polyester, and chlorinated rubber. Each has its own advantages and disadvantages. When using these materials, it is particularly important that you exactly follow the manufacturers instructions.
SOLVENTS, OR THINNERS. The only purpose of a solvent, or thinner, is to adjust the consistency of the material so that it can be applied readily to the surface. The solvent then evaporates, contributing nothing further to the film. For this reason, the cheapest suitable solvent should be used. This solvent is likely to be naphtha or mineral spirits. Although turpentine is sometimes used, it contributes little that other solvents do not and costs much more.
NOTE Synthetic resins usually require a special solvent. It is important the correct one be used; otherwise, the paint may be spoiled entirely.
Paints, by far, comprise the largest family of structural coatings you will be using to finish products, both interior and exterior. In the following section, well cover some of the most commonly encountered types.
OIL-BASED PAINTS. Oil-based paints consist mainly of a drying oil (usually linseed) mixed with one or more pigments. The pigments and quantities of oil in oil paints are usually selected on the basis of cost and their ability to impart to the paint the desired properties, such as durability, economy, and color. An oil-based paint is characterized by easy application and slow drying. It normally chalks in such a manner as to permit recoating without costly surface preparation. Adding small amounts of varnish tends to decrease the time it takes an oil-based paint to dry and to increase the paints resistance to water. Oil-based paints are not recommended for surfaces submerged in water.
ENAMEL. Enamels are generally harder, tougher, and more resistant to abrasion and moisture penetration than oil-based paints. Enamels are obtainable in flat, semigloss, and gloss. The extent of pigmentation in the paint or enamel determines its gloss. Generally, gloss is reduced by adding lower cost pigments called extenders. Typical extenders are calcium carbonate (whiting), magnesium silicate (talc), aluminum silicate (clay), and silica. The level of gloss depends on the ratio of pigment to binder.
EPOXY. Epoxy paints area combined resin and a polyamide hardener that are mixed before use. When mixed, the two ingredients react to form the end product. Epoxy paints have a limited working, or pot, life, usually 1 working day. They are outstanding in hardness, adhesion, and flexibility-plus, they resist corrosion, abrasion, alkali, and solvents. The major uses of epoxy paints are as tile-like glaze coatings for concrete or masonry, and for structural steel in corrosive environments. Epoxy paints tend to chalk on exterior exposure; low-gloss levels and fading can be anticipated. Otherwise, their durability is excellent.
LATEX. Latex paints contain a synthetic chemical, called latex, dispersed in water. The kinds of latex usually found in paints are styrene-butadiene (so-called synthetic rubber), polyvinyl acetate (PVA or vinyl), and acrylic. Latex paints differ from other paints in that the vehicle is an emulsion of binder and water. Being water-based, latex paints have the advantage of being easy to apply. They dry through evaporation of the water. Many latex paints have excellent durability. This makes them particularly useful for coating plaster and masonry surfaces. Careful surface preparation is required for their use.
RUBBER-BASED. Rubber-based paints are solvent thinned and should not be confused with latex binders (often called rubber-based emulsions). Rubber-based paints are lacquer-type products and dry rapidly to form finishes highly resistant to water and mild chemicals. They are used for coating exterior masonry and areas that are wet, humid, or subject to frequent washing, such as laundry rooms, showers, washrooms, and kitchens.
PORTLAND CEMENT. Portland cement mixed with several ingredients acts as a paint binder when it reacts with water. The paints are supplied as a powder to which the water is added before being used. Cement paints are used on rough surfaces, such as concrete, masonry, and stucco. They dry to form hard, flat, porous films that permit water vapor to pass through readily. When properly cured, cement paints of good quality are quite durable. When improperly cured, they chalk excessively on exposure and may present problems in repainting.
ALUMINUM. Aluminum paints are available in two forms: ready mixed and ready to mix. Ready-mixed aluminum paints are supplied in one package and are ready for use after normal mixing. They are made with vehicles that will retain metallic brilliance after moderate periods of storage. They are more convenient to use and allow for less error in mixing than the ready-to-mix form.
Ready-to-mix aluminum paints are supplied in two packages: one containing clear varnish and the other, the required amount of aluminum paste (usually two-thirds aluminum flake and one-third solvent). You mix just before using by slowly adding the varnish to the aluminum paste and stirring. Readyto-mix aluminum paints allow a wider choice of vehicles and present less of a problem with storage stability. A potential problem with aluminum paints is moisture in the closed container. When present, moisture may react with the aluminum flake to form hydrogen gas that pressurizes the container. Pressure can cause the container to bulge or even pop the cover off the container. Check the containers of ready-mixed paints for bulging. If they do, puncture the covers carefully before opening to relieve the pressure. Be sure to use dry containers when mixing aluminum paints.
In contrast to paints, varnishes contain little or no pigment and do not obscure the surface to which applied. Usually a liquid, varnish dries to a hard, transparent coating when spread in a thin film over a surface, affording protection and decoration.
Of the common types of varnishes, the most important are the oils, including spar, flat, rubbing, and color types. These are extensively used to finish and refinish interior and exterior wood surfaces, such as floors, furniture, and cabinets. Spar varnish is intended for exterior use in normal or marine environments, although its durability is limited. To increase durability, exterior varnishes are especially formulated to resist weathering.
Varnishes produce a durable, elastic, and tough surface that normally dries to a high-gloss finish and does not easily mar. Often, a lower gloss may be obtained by rubbing the surface with a very fine steel wool. However, it is simpler to use a flat varnish with the gloss reduced by adding transparent-flatting pigments, such as certain synthetic silicas. These pigments are dispersed in the varnish to produce a clear finish that dries to a low gloss, but still does not obscure the surface underneath (that is, you can still see the grain of the wood).
Shellac is purified lac formed into thin flakes and widely used as a binder in varnishes, paints, and stains. (Lac is a resinous substance secreted by certain insects.) The vehicle is wood alcohol. The natural color of shellac is orange, although it can be obtained in white. Shellac is used extensively as a finishing material and a sealant. Applied over knots in wood, it prevents bleeding.
Lacquers may be clear or pigmented and can be lusterless, semigloss, or glossy. Lacquers dry or harden quickly, producing a firm oil- and water-resistant film. But many coats are required to achieve adequate dry-film thickness. It generally costs more to use lacquers than most paints.
Stains are obtainable in four different kinds: oil, water, spirit, and chemical. Oil stains have an oil vehicle; mineral spirits can be added to increase penetration. Water stains are solutions of aniline dyes and water. Spirit stains contain alcohol. Chemical stains work by means of a chemical reaction when dissolved by water. The type of stain to use depends largely on the purpose, the location, and the type of wood being covered.