Lesson 2-3 PREPARING THE SURFACE
2-9. Surface Preparation
Good surface preparation is essential. Different surfaces, such as a nail hole in wood trim or cracks in a plaster wall, require different treatment and materials. Several materials are available to improve the surface before painting. They are undercoat, sealer, primer, size, wood filler, plastic wood, caulking compound, putty, texture paint, spackling compound, joint cement, and paint and varnish remover.
a. Undercoat. An undercoat is the coat applied prior to the finishing or the final coat. On old work, the undercoat is the first coat applied; on new work, the undercoat is applied after the primer and before the final coat. Undercoat is essential for its hiding power, and it provides a smooth surface for the final coat.
b. Sealer. A sealer fills the pores of absorbent surfaces in preparation for painting. By preventing the absorption of oil from the paint, a sealer reduces the number of coats of paint required and ensures a stronger film. Some sealers are used to protect the final coat from chemicals that are present in the surface. Sealers are usually applied as the first coat. There are many commercial sealers. One type that you can easily make is a mixture of shellac and alcohol. Reduce the shellac with alcohol, 4 to 1 if it is to be used over a filler and 7 to 1 if it is to be used over stain. Then, pour the mixture into an equal amount of lacquer to make a sealer.
c. Primer. A primer is a film-forming material used as the first coat of paint. It provides adhesion for the following coats and prevents undesirable chemical reaction to the surface. It may be a thinned first coat of paint or a specially prepared product. Different types of primers are available for different types of surfaces. Exterior wood primer, epoxy primer, phenolic-resin primer-sealer, wash primer, zinc-chromate primer, and metal primer are discussed below:
(1) Exterior wood primer. A wood primer is usually a thinned coat of the paint to be used or a specially prepared coat.
(2) Epoxy primer. An epoxy primer is a two-component pigment primer specially made to stick well and reflect heat. It is nonchalking, nongloss, and very resistant to chemicals, lubricants, and corrosive atmospheres. This primer is made primarily for spraying; however, you may brush it on small areas. Mix it by adding equal parts, by volume, of the pigmented primer to the catalyst just before using. Use epoxy primer within 10 hours after mixing. Because it is an epoxy, this primer is difficult to remove. There are several types of epoxy primers designed to be used on specific items. For this reason, you should follow the manufacturers recommendations when using them.
(3) Phenolic-resin primer-sealer. A phenolic-resin primer-sealer is a new type of finish that is well suited for open-grain woods. It penetrates into the pores of the wood, dries, and equalizes the density of the hard and soft grains. Staining and painting of wood thus treated eliminates light and dark streaks that are frequently present on untreated wood. This sealer is light amber in color and almost as thin as water. You can add pigment as well as color in oil to produce almost any color desired.
(4) Wash primer. The term wash primer designates a specific material that combines the properties of an inhibitive wash coat or metal conditioner with the properties of a conventional anticorrosive primer. The essential components of wash primers are phosphoric acid, zinc chromate pigment, and polyvinyl butyral resin. Wash primers can be formulated so that they are equally effective on iron, steel, aluminum, treated magnesium, copper, zinc, and a wide variety of other metals. Discard mixed primer after 8 hours. Many coatings adhere well to the wash primer, including oil-based alkyds, epoxies, and urethanes. Wash primers—
- Apply easily and dry rapidly.
- Are usable in wide ranges of temperatures and humidity levels.
- Can be applied to a variety of metals with good results.
- Provide temporary protection until a protective coating is applied.
- Prevent or retard corrosion development.
- Exhibit a high degree of adhesion to metals.
- Upgrade the performance of subsequent protective coatings.
(5) Zinc-chromate primer. Zinc-chromate primer is used in tremendous quantities by the armed forces. Its rust-inhibitive qualities prove very satisfactory. Do not use it in a straight linseed-oil vehicle but in a synthetic resin vehicles, such as phenolic resin or alkyd resin. For priming structural steel, the addition of some raw linseed oil is advantageous. Use zinc-chromate primer as a primer for metal surfaces, such as structural steel, bridges, tanks, refrigerators, railroad cars, motor vehicles, and aircraft.
(6) Metal primer. Pigments of lead and chromate are considered hazardous; as a result, two other primers have replaced red lead-based primer They are zinc-molybdate primer and red iron-oxide, zinc-oxide, linseed-oil primer. These primers do not contain toxic lead or chromate pigments, but some corrosion protection has been sacrificed.
(a) Zinc-molybdate primer. Zinc-molybdate primer (alkyd type) is designed for use on steel (cleaned to a commercial-blast grade or better) and aluminum. This primer has low VOC content.
(b) Red iron-oxide, zinc-oxide, linseed-oil primer. This primer replaces red lead-based primer. It is designed for use on steel that has been cleaned with hand tools.
d. Size. Size is used to fill the pores of plaster or wallboard so that paint will stay on the surface. There are several types of size available, but the main types are glue-water and thinned varnish. Prepare varnish size by thinning 1 gallon of varnish with 1 quart of turpentine. Prepare glue-water size by mixing glue and water until the mixture will spread easily. Primers and sealers have been improved to the extent that size is seldom needed for paint preparation today.
e. Wood Filler. Wood filler provides a smooth, even finish on wood that has open grain, such as walnut, mahogany, and oak. Paste wood fillers are usually made of silex or silica ground in linseed oil and lacquer drier with various pigments added for color. Fillers come in standard colors, such as mahogany, light oak, dark oak, maple, walnut, black, white, and natural.
Most fillers come in paste form and are too thick to be used without thinning. Thin filler by adding turpentine, naphtha, or a special thinner according to the instructions on the container. About 12 pint of thinner to 1 pound of filler is required for coarse, open-grained woods. Closer grained woods, such as walnut, rosewood, mahogany, and zebrawood, require a mixture of about 10 ounces of thinner to 1 pound of filler. When you are mixing filler, add a little thinner at a time until you can get the desired consistency.
f. Plastic Wood. Plastic wood is used to fill holes in wood, especially if the finish is to be clear. It is available commercially and can be purchased in various quantities. This filler dries very rapidly and must be kept in airtight containers.
g. Caulking Compound. Caulking compound is used to seal joints around doors and windows and between baseboards and wallboards. It usually contains asbestos fiber, a pigment for opacity, fish or soybean oil, and a drier. Caulking compound will remain elastic for some time; this allows it to expand and contract with the movement of the building. There are two main types of caulking compound: the gun type, which is forced into cracks and crevices in ribbonlike form, and the knife type, which is applied with a putty knife. Caulking compound can also be used as a window-sash putty, and it is available in rolls or strips that are applied by pressing them into joints.
h. Putty. Putty is used to fill holes in surfaces and to replace checked or broken putty around windows. Commercial putty is made of white alkyd and whiting combined with linseed oil and a neutral oil to prevent it from drying too rapidly. Occasionally, you may substitute plastic wood for putty when filling holes in wood.
i. Texture Paint. Texture paint has a heavy consistency that is designed to produce a textured effect on a surface. Since it is thick and can be molded to obtain various decorative effects, it is particularly suitable for finishing sheetrock in drywall construction. You can make texture paint on the job by mixing joint cement with paint to obtain a butter consistency. Mix the color before applying the paint.
j. Spackling Compound. Spackling compound is a white powder. When mixed with water, it sets quickly without swelling or shrinking. Use spackling compound to apply a texture effect on smooth interior surfaces, such as plaster, sheetrock, wallboard, gypsum board, and primed and unprimed wood.
k. Joint Cement. Joint cement is used to fill depressions left by a hammer when nailing sheetrock. You may also use it with perforated tape to fill the recessed edges of sheetrock joints. Mix joint cement with water until a thick paste is formed, and apply it with a broad-blade putty knife or a cement trowel.
l. Paint and Varnish Remover. Paint and varnish remover is made of chemical solvents that are spread over the old finish to soften it so that it can be removed with a steel scraper, a putty knife, or steel wool. It is available in paste or liquid form; the liquid form is faster-acting.