Wood Finishing Stains and Paints for Exterior Siding

Textured plywood surfaces are probably the most common for exterior siding. Sanded and rough sawn plywood will develop surface checks, especially when exposed to moisture and sunlight. These checks, coupled with the flat grain pattern (wide bands of dark, dense latewood) characteristic of nearly all plywood, can lead to early paint failure. These paint failures can be minimized by the use of top quality acrylic latex paint systems.

Semitransparent Stains

Unlike paints, semitransparent penetrating oil-base stains cannot check and peel from plywood surfaces. These stains penetrate the wood and do not form a continuous film or coating like paint. Semitransparent penetrating stains allow most of the wood grain to show through, and the color can be controlled by pigments added to the stain. Penetrating stains also perform well on weathered surfaces. New, smooth surfaces may also be stained. Oil-base penetrating stains have a longer life expectancy when properly applied to rough sawn or weathered surfaces.

Semitransparent stains may be brushed or rolled on. Brushing should give better penetration and performance, especially on textured surfaces. These stains are generally thin and runny, so an application can be a little messy. Lap marks will form if stains are improperly applied. Lap marks can be prevented by staining only a small number of boards or a panel at one time. Working in the shade is desirable because the drying rate is slower. The penetrating stain should be stirred frequently during the application. One gallon will usually cover about 300-400 square feet of smooth surface and from 150-250 square feet of the rough surface.

For long life with penetrating oil-base stain on rough sawn or weathered lumber, use two coats and apply the second coat before the first is dry. Apply the first coat to a panel or area as you would to prevent lap marks. Then work on another area so the first coat can soak into the wood for 20 to 60 minutes. Apply the second coat before the first coat has dried. (If the first dries completely, it may seal the wood surface so that the second coat cannot penetrate the wood). About an hour after applying the second coat, use a cloth or sponge to wipe off the excess stain that has not penetrated the wood. A stain that did not penetrate may form an unsightly surface film and glossy spots.

Note.- Sponges or cloths that are wet with oil-base stain are particularly susceptible to spontaneous combustion. To prevent fires, bury them, immerse them in water, or seal them in an airtight container immediately after use.

A two coat wet system on rough wood may last as long as 10 years. If only one coat of penetrating stain is used on new wood, its expected life is 2-4 years, but succeeding coats will last longer.

Refinishing semitransparent penetrating oil-base stains is relatively easy. Excessive scraping and sanding are not required. Simply use a stiff bristle brush to remove all surface dirt, dust, and loose wood fibers, and then apply a new coat of stain. The surface should be free of mildew. A longer service life can be expected for penetrating stains the second time they are applied since they will penetrate the many small surface checks which open up as wood weathers.

Water-repellent preservatives can also be used as a natural finish for plywood. Water-repellent preservatives are mixtures of a solvent such as mineral spirits or other paint thinners, wax, resin or drying oil and a wood preservative. These finishes, like semitransparent stains, penetrate the wood and do not form a surface film, so peeling will not be a problem. Since they do not contain any coloring pigments, they will allow the natural wood color and grain to show through. Expected service life is only 1 to 2 years, and frequent reapplication is necessary to protect the wood surface.

Water-repellent preservatives are best applied by dipping, but brush treatment to the point of refusal is also satisfactory. It is especially important to apply liberal amounts of the solution to all joints or other potential places where moisture might accumulate. Be certain to treat the horizontal bottom edges of any panels.

Refinishing water-repellent preservatives is accomplished by simply cleaning the old surface with a bristle brush and applying a new coat of finish. To determine if a water-repellent preservative has lost its effectiveness, splash a small quantity of water against the wood. If the water beads up and runs off the surface, the treatment is still effective. If the water soaks in, the wood needs to be refinished. Refinishing is also required when the wood surface shows signs of graying. Water repellents are sometimes used in the same manner as water-repellent preservatives. However, they do not contain a wood preservative and will not protect against surface mold and mildew.

Note.: Steel wool and wire brushes should not be used to clean surfaces which will be finished with semitransparent stains or water-repellent preservatives. Small iron deposits left on the surface can react with certain wood extractives to form a dark-blue, unsightly discoloration that is sealed beneath the new finishing system.

Paints

In some cases, the painting of plywood is required or desirable. Top-quality acrylic latex paints are the best choice for exterior surfaces. For overlaid or MDO plywood, remove all loose paint with a stiff bristle brush and then scrub with a soft brush or sponge and water. Rub your hand against the cleaned surface to determine if any residues remain. When necessary, scrubbing with a detergent or paint cleaner will usually remove additional residues. Then rinse well and allow to dry before repainting.

If non-overlaid plywood is to be painted, follow these tips. First, brush a liberal quantity of water- repellent preservative or water repellent onto all the edges of the plywood sheets. The surface should also be treated in the same manner. The water repellent will help reduce wood's tendency to absorb moisture through the end grain and surface lathe checks. Allow the water-repellent preservative or water repellent to dry for at least two warm days. Then prime the plywood surface with a high-quality paint recommended for use on woods which contain extractives. The primer should be applied thick enough to obscure the wood grain pattern. Two coats of a high-quality acrylic latex house paint should be applied over the primer. Allow at least two days but no longer than two weeks between the primer and topcoat. The primer and topcoat should be compatible and preferably from the same manufacturer. Always remove the mildew before refinishing.

Refinishing painted plywood requires proper surface preparation if the new paint coat is to give the expected performance. First, scrape away all loose paint. Use sandpaper on any remaining paint to "feather the edges" smooth with the bare wood. Then scrub the remaining paint with a brush or sponge and water. Household bleach (50/0 sodium hypochlorite) used at the rate of 1 cup of bleach to 3 cups of water will remove mildew. Rinse the surface with clean water. Wipe the surface with your hand. If the surface is still dirty or chalky, scrub it again using a detergent or paint cleaner. Rinse the surface thoroughly with clean water, and allow it to dry before repainting. Areas of exposed wood should be treated with a water-repellent preservative or water repellent and allowed to dry for at least two days and then primed. One or preferably two topcoats should follow.

Reconstituted wood products are those made by forming large sheets, usually 4 by 8 feet, from small pieces of wood or pulp. Reconstituted wood products may be smooth or textured to look like standard lumber. Depending upon the basic wood component used in their manufacture, reconstituted wood products may be classified as fiberboard or particleboard.

Reconstituted wood products can be purchased unfinished, primed, with a topcoat, or stained. Only some fiberboards and particleboards are manufactured for exterior use. Be sure to check with the supplier, and follow the manufacturer's directions in using and finishing these board type products. Film-forming finishes such as acrylic latex paints and solid color acrylic latex stains will give the most protection. Other finishes such as semi-transparent stains allow a more rapid deterioration of the surface to take place.

To paint hardboard and particleboard, follow good finishing practices as recommended for plywood. Be sure to seal all edges with a water-repellent preservative or water repellent. The surface should be treated with the same solution. Then apply a primer coat recommended for use over wood, followed by at least one high-quality top coat of acrylic latex house paint.

Some reconstituted wood products may be factory primed with paint and some may even have a topcoat. Factory primed boards should not be allowed to weather for more than a couple of weeks before top coating. If excessive weathering does occur, clean the factory primed surface, and then reprime and follow with two topcoats.

Reconstituted wood products may be refinished by following those procedures recommended for plywood. Care must be exercised in preparing a wood floor since finishing will accentuate any defects, irregularities or roughness. Even irregularities that can scarcely be seen before finishing become conspicuous afterward. Nothing can be done later which will make up for the defects of a poor sanding job. Unless prefinished, flooring usually has not been sanded by the manufacturer and bears across- the-grain ridges left by planing. These marks will mar the appearance when the finish is applied. Moreover, if much time passes between the final sanding and the application of finish, some roughness may develop from raising of the wood grain because of changing moisture content. Floors should be sanded shortly before finishing is begun.

Materials here are from North Central Regional bulletin Wood Finishing, Finishing Exterior Plywood, Hardboard, and Particleboard, with Purdue Extension and USDA Forest Service.