Question: How would you recommend refinishing old plywood siding? How often should the plywood be refinished to best maintain the siding?

Answer: Any finish will begin to show its age after prolonged exposure to sun and weather. The weathering process can gradually erode the finish, cause it to become brittle and crack, lose its adhesion, fade, or mildew. Where the finish has weathered away, sunlight and moisture will erode and roughen the surface. For continuing satisfactory performance, it is important to maintain the finish. It is also important, however, not to refinish too often, especially when oil-based solid-color stains are used. If too many coats of an oil-based finish are applied, the finish becomes too brittle and will fail prematurely.

Finishes should be renewed when areas of unprotected bare wood first become visible or when obvious deterioration of the finish, such as peeling or flaking, is noticed. These types of finish failures usually occur first on the south and west sides of the building. 

SURFACE PREPARATION – Thorough surface preparation is essential. Any finish applied over dirt, mildew, chalk, loose finish, or weathered wood that has not been cleaned of debris will not last long. 

REMOVING MILDEW – Mildew can develop on almost any type of finish, especially in warm, humid conditions or in areas with poor air circulation. It usually begins as dark spots which look like surface dirt. Unlike dirt, it disappears when household bleach is applied. In severe cases it may cause a uniform gray or black discoloration on large areas. Mildew must be killed and removed before refinishing or it may continue to grow through the newly applied finish.

Mild cases of mildew can be removed by scrubbing the surface with a commercial mildew-removing product or a solution of one part household bleach mixed in three parts, by volume, of warm water. Then rinse well with water. More severe cases might require repeated applications. When using bleach, wear rubber gloves and use goggles to prevent eye damage.

REMOVING CHALK, DIRT AND LOOSE WOOD FIBERS – Remove chalk and dirt by washing with a household detergent solution. Rinse well to remove the detergent. A stiff nonmetallic brush can be used for rapid, thorough cleaning. Avoid wire brushes since broken off metal fragments can cause staining. When loose wood fibers are present due to weathering, brush first across the grain to remove the fibers and then along the grain to remove any brush marks. 

REMOVING LOOSE FINISH – If the finish shows evidence of flaking or peeling, remove loosened particles with a stiff nonmetallic bristle brush. Wetting the surface prior to brushing sometimes helps. For more stubborn cases, a water blaster, available from most rental equipment agencies, can be used. When using the water blaster, avoid forcing water into the wall around window casings and siding joints, and also avoid very high pressures which may damage the wood surface. A water-rinsable type of paint remover can also be used to remove opaque finishes if deterioration is advanced. Carefully follow all manufacturers recommendations when using paint remover products.

When removing loose finish from overlaid sidings, be careful not to damage the overlaid surface. A stiff bristle brush or water-rinsable paint remover normally works best on overlaid sidings.
After removing any loosened finish, be sure to wash the surface thoroughly prior to refinishing.

TESTING THE SURFACE – The suitability of a cleaned surface for refinishing is easily checked with adhesive tape. Apply the tape with finger pressure to a small area of cleaned and repainted surface. If the tape does not remove the finish when it is removed by a quick pull, the surface is satisfactory for repainting. 

THE NEW FINISH – The clean siding is refinished just as if it were new. Typically, one coat is satisfactory, except where opaque finishes are applied over bare wood. Bare wood areas should be spot-primed. Also, when making color changes, two coats may be necessary to hide the old color. The selection of the new finish will depend upon the appearance desired as well as the siding’s condition and the appearance of the old finish. Reference APA publications: Finishing APA Rated Siding for Protection and Appearance, Form No. M335, and Finishing APA Rated Siding, Form No. Q350, for recommendations for suitable finishes.

Because weathered wood surfaces often require more finish than new wood, allow for more material when refinishing. A proper application rate will give longer life and greater protection. Check the paint manufacturer’s recommendations for suitable application rates.

Refinishing helps to obscure face-checking that may occur on the veneer surfaces during weathering. The new finish should be thoroughly worked into the face-checks with a brush. If the finish is spray-applied, back-brush the finish into the face-checks. Face-checking is least noticeable when flat earth-tone colors are used.

Stains and paints differ in appearance and performance. Always select a top-quality product formulated for wood and apply it correctly to help assure lasting performance and beauty. And finally, selection of high-quality equipment will help assure proper application of the finish. Check with paint supplier for recommendations. 

APPLICATION METHODS – The application method is as important as the finish material itself. It is poor economy to buy a quality stain or paint and then cause it to fail prematurely because of improper application.

For best performance and a uniform appearance, be sure that the finish is worked well into any bare wood. It’s best to apply the finish with a brush. If a spray is used, the finish should be applied liberally and then back-brushed or back-rolled while it is still wet. Spraying alone does not work the finish into a textured bare wood surface. Back-brushing or back-rolling also helps to even out spray patterns, giving a more uniform appearance. If spray is fogged onto the siding, too little is applied and it adheres only to the outer surface of loose dust and fibers. These quickly erode in the natural weathering process, causing premature failure of the finish. Long-napped paint rollers may also be used, but it is important that the proper amount of finish be applied and that the finish be worked well into the wood surface. Final brushing should be along the grain to erase any marks. Back-brushing may not be necessary when refinishing over a properly prepared surface and the existing solid-color stain or paint system is in good condition. Oil-based semitransparent stains should always be back brushed when applied to plywood siding surfaces.

If the siding is very dry, application and performance of a latex finish is improved if the surface is dampened first. The surface should not be moistened when using oil-based products, however.
Do not paint or stain in the rain, when temperatures are low, or in direct sunlight when the panel is hot. Minimum temperatures are commonly 50°F (10°C) for latex and 40°F (5°C) for oil systems. Check with the coating manufacturer for specific recommendations.

Use only top-quality tools and equipment in preparing and finishing all surfaces. Check with paint suppliers for recommendations on application equipment.

Regardless of the method of application, follow the coverage rate recommended by the finish manufacturer. For textured plywood, up to twice as much finish is needed as for sanded plywood. Be sure to finish complete sections at one time in order to avoid lap marks.