Before applying paint, the surface must be clean, dry, and dull. Clean means there should be no dirt, dust, oil, loose surface, peeling paint, mildew, or other contaminants on the surface. Dry means the moisture content for the substrate should be less than 8 percent for wood, less than 12 percent for plaster, and less than 15 percent for masonry. Dull means the surface should have tooth, from sanding or applying deglosser. Composition hardboard must be cleaned thoroughly with a solvent prior to painting. Plaster must be allowed to dry thoroughly in a warm room for at least 30 days before painting.
Previously coated surfaces require special attention. If a surface has been previously painted, the old paint may have been applied before 1978, in which case care will need to be taken when sanding due to a possibility of lead in the sanding dust. Usually, a dust containment system will be required around the work area. All surface contamination such as oil, grease, loose paint, dirt, foreign matter, rust, mold, mildew, mortar, efflorescence, and sealers must be removed to assure sound bonding of a new coating to the existing coating. Glossy surfaces of old paint must be clean and dull before repainting. Any bare areas need to be primed before painting.
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The Painting and Decorating Contractors of America publish a document titled Levels of Surface Preparation for Repainting and Maintenance Projects Receiving Architectural Coatings, PDCA P14-06.
Paint can be applied with a sprayer, a roller, or a brush. Paint application is measured in mils: 1 mil is 1/1,000 inch. The thickness of the paint should be approximately 3 to 4 mils per wet coat. The coverage will depend on the texture of the surface, the preparation of the surface, and the method of application. For estimation purposes, many architectural paints are applied at 300 to 450 square feet per gallon of paint.
For the majority of interior applications, paint will be applied to a gypsum board wall or ceiling. The surface will be 1/2-inch thick gypsum board, nailed to wood or metal vertical studs. Make sure the surface is uniformly smooth, all nails set, nail holes filled, joints taped, joint compound applied, and sanded smooth. Dust from sanding must be removed prior to painting. Gypsum board must be primed prior to applying the finish color paint. Flat or satin paint is normally applied to gypsum board.
The second most common application will be on wood trim. Nail holes need to be filled before painting. If the wood is unfinished, it will need to be sealed with a primer or sealer. For staining wood trim, wood putty is used to fill nail holes so it will take the color of the stain. A protective finish of varnish is applied over the stain. For painting wood trim, wood sealer or primer may be used under paint, and nail holes may be filled with spackle or putty. Paint for wood trim is often applied with a brush of bristles or sponge. A heavy application is desirable on wood trim, so spray application is not normally used. Normally, semi-gloss or gloss paint is used.