Getting a quality exterior paint job on your wood siding can be tricky. Weather, dust, and incorrect preparation can all cause the result to suffer if you aren't careful. If you plan to repaint your home's exterior, read on for more information on how to do it right so you don't have to repaint it again so soon.
Note: Other types of exteriors will have different techniques to get the best results.
Types of Paint
Paints are common coatings used on wood. Latex-based paints and stains are water borne, and oil or alkyd paints are solvent borne. Paints are used for aesthetic purposes, to protect the wood surface from weathering, and to conceal certain defects. When they are applied to the wood, they do not penetrate deeply like stain; rather, the wood grain is completely obscured, and a surface film is formed. This surface film can blister or peel if the wood is wet or if inside water vapor moves through the house wall and wood siding because of the absence of a vapor barrier.
Latex paints are generally easier to use since water is used in clean-up. They are also porous and, thus, will allow some moisture movement. In comparison, oil-based paints require organic solvents for clean-up, and some are resistant to moisture movement. Paints are the only way to achieve a bright white finish. However, they, especially oil-based paints, are subject to peeling where moisture is a problem.
Prepare the Surface for a Lasting Look
Remember that a coat of fresh paint will not glue down a coat of old paint that's ready to crack and fall off. If a new coat is to be successful over a problem area, any issues must be eliminated. Good surface preparation is also essential. If the wood has been completely stripped or is being painted for the first time, it should be brush treated with a paintable water-repellent preservative or water repellent. This treatment will reduce the uptake of moisture by the wood. After this solution has dried for at least two warm days, following manufacturers specifications, apply an oil-based primer or a stain-blocking latex primer, followed by at least one top coat of high-quality acrylic latex paint. One top coat of paint should last four to five years, but two top coats can last up to eight or 10 years.
If inter-coat peeling is a problem, clean and prepare the entire surface as mentioned before; then, apply the desired top coat. Where inter-coat peeling has happened, it is particularly important to clean areas protected from sun and rain such as porches, eaves and side walls protected by overhangs. It is probably adequate to repaint these protected areas every other time the house is painted. If paint fails because of penetration of rain and dew through porous paint, clean and prepare the paint surface.
If only isolated areas of the paint coat have failed by peeling to the bare wood, spot painting may be the best alternative. Begin by scraping away all loose paint. Then sandpaper or "feather" the edges of any remaining paint to smooth it with bare wood. Clean the old painted surface by scrubbing with a sponge or bristle brush, and then rinse it with clean water. Test the the surface with your hand afterward to see if it's ready. If the surface is still dirty or chalky, scrub it again using a detergent, and rinse it with clean. After the old paint and wood has been thoroughly dried, apply one coat of a paintable water-repellent preservative or water repellent to the bare patches, being careful to liberally treat end and lap joints. Any repellent on the painted surface should be wiped dry with rags, and at least two warm days should be allowed for the repellent to dry before painting.
All too often, exterior finishes for wood are short lived in durability or fail completely as a result of selecting the wrong kind of finish or of not following recommended application procedures.
Types of Wood Products
Each product has unique characteristics which will affect the durability of any finish applied to it. Dimensional change in lumber occurs as the wood gains or loses moisture. Wood in heated homes tends to dry and shrink in the winter and gains moisture and swells in the warm summer months. Excessive dimensional change in wood consistently stresses a paint film and may result in early failure.
Grain direction affects paint-holding characteristics and is determined at the time lumber is cut. Edge-grained bevel siding will hold paint well while flat-grained lumber will not since it shrinks and swells more than edge-grained lumber and because wide, dark bands of summer wood are frequently present. Although penetrating stains or preservative treatments are best on rough sawn lumber, paint will actually last longer on smooth, edge-grained surfaces. Stain treatments often accentuate the natural or rustic look of rough sawn lumber and allow the wood grain and surface texture to show through the finish.
Sanded and rough sawn plywood will develop surface checks, especially when exposed to moisture and sunlight. These surface checks can lead to early paint failure with oil or alkyd paint systems. Plywood manufactured with a medium density paper overlay, frequently called MDO, holds paint well when compared to standard rough sawn or even smooth plywood. MDO plywood is not always a stock item in many lumber yards, but it can usually be ordered.
An oil-based primer or stain-blocking latex primer should be applied to the bare wood. After that, your should make sure to caulk all cracks and openings on your wood surface.
Two coats of a good-quality acrylic latex house paint should be applied over the primer. If it is not practical to apply two top coats to the entire house, consider two top coats for fully exposed areas on the south and west sides. Areas fully exposed to sunshine and rain are the first to deteriorate and therefore should receive two coats.
One gallon of paint will cover about 400 square feet of flat surface area. However, coverage can vary with different paints and application procedures. Research has indicated that the optimum thickness for the total paint coat (primer and two top coats) is four to five mils or about the thickness of a sheet of newspaper. The quality of paint is usually, but not always, related to the price. It should also be noted that proper brush application is always superior to roller application.
To avoid future separation between paint coats, the first top coat should be applied within two weeks after the primer and the second coat within two weeks of the first. As certain paints weather they can form a soap-like substance on their surface which may prevent proper adhesion of new paint coats. If more than two weeks elapse before applying another paint coat, scrub the old surface with water using a bristle brush or sponge. If necessary, use a mild detergent to remove all dirt and deteriorated paint. Then rinse well with water, and allow the surface to dry before painting.
To avoid temperature blistering, oil-based paints should not be applied on a cool surface that will be heated by the sun within a few hours. Temperature blistering is most common with thick paint coats of dark colors applied in cool weather. The blisters usually show up in the last coat of paint and occur within a few hours to one or two days after painting. They do not contain water. Oil-based paint may be applied when the temperature is 40 degrees Fahrenheit or above. A minimum of 50 degrees Fahrenheit is desired for applying latex-based paints, and for proper curing, the temperature should not drop below 50 degrees for at least two hours after paint application. Low temperatures will result in paint failure. To avoid wrinkling, fading, or loss of gloss with oil-based paints and streaking of latex paints, paint should not be applied in the evenings of cool spring and fall days where heavy dew can form before the surface of the paint has thoroughly dried.
This article has been contributed in part by Michigan State University Extension.