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Q.Is it okay to use an exterior latex house paint over stained wood siding?

A.Bill Feist, a former wood-finishes researcher with the Forest Products Laboratory in Madison, Wis., and co-author of Finishes for Exterior Wood, responds: That should work fine, as long as the substrate is properly nailed and in good condition and there are no obvious moisture problems.

Treat the old finish the way you would any coat of paint: Remove loose and peeling finish, then sand the surface, taking care to feather sharp edges left over from scraping. Any bare wood should be sanded, too. And if the old surface is shiny or glossy, sand it or treat it with a deglosser so the new paint will adhere.

As usual, the siding must be cleaned to remove chalking, oils, mildew, and other contaminants. The best approach — though labor-intensive — is to scrub all of the siding with a detergent-and-water mixture and then rinse it thoroughly. Simple power washing can also be effective, but if there is a mildew-and-dirt problem, spray first with detergent and rinse, then spray with mildew cleaner (usually a bleach) and rinse again.

After everything has dried, the surface can be recoated. Use an all-acrylic paint. If the old finish is an opaque oil-based stain, apply a coat of alkyd (oil-based) primer to ensure good adhesion and provide a base for the top coat (two coats are preferable).

If the original finish is a solid-color latex stain that adheres firmly to the wood, it probably isn't necessary to prime the finish before painting, though all of the prep work is still important.

One caution: Some semitransparent and transparent penetrating-oil finishes contain water repellent that can interfere with the adhesion of the new top coat (whether paint or opaque stain). These finishes should be thoroughly cleaned with a strong detergent, and then rinsed and dried.

In really troublesome cases, the old stain may have to be removed (stain removers are available from a number of deck-stain manufacturers). If you have to do this, I'd recommend using two coats of an alkyd primer to provide a good base for future paint.