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Q.How should paints and stains perform on pressure-treated wood? I haven’t had a lot of luck painting the PT wood decks I build: The paint seems to fail after only two or three years. Is this because pressure-treated lumber is so wet, or do the chemicals used to treat the wood cause the paint to come off?

A.William Feist responds: It’s not surprising that the paint on your pressure-treated wood decks is failing in two or three years. Normally, decks should not be painted, but should be finished regularly (annually or biannually) with a penetrating semitransparent stain or a penetrating clear finish (especially those finishes designed for use on decks). Paints and solid-color stains simply cannot hold up to the severe exposure of a deck surface. As small cracks develop on the deck’s surface from exposure to sun and water, water passes through and the paint soon peels. Penetrating finishes cannot fail in this way because they do not form a film, so they are much more suitable for decks. In addition, the wood species used for pressure treating is usually southern pine, which does not hold paint well because it tends to expand and contract a lot.

Studies at the U.S. Forest Products Laboratory in Madison, Wis., have shown that semitransparent stains and clear finishes will actually last longer on pressure-treated wood (CCA, or chromated copper arsenate). This is because the chromium in the treatment protects the wood surface from ultraviolet degradation. Paints and solid-color stains will perform well on pressure-treated wood that is used in an upright position (on fences, for instance), but only when the wood has been cleaned and is thoroughly dry before painting.

William Feist is a consultant and teacher on wood weathering and exterior wood finishing. He was a research chemist at the Forest Products Lab in Madison, Wis., for 30 years.