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Q.Is it a good idea to pressure-wash wood siding before painting? It seems like this would drive a lot of water into and behind the siding — water that might still be present when the paint is applied. And what about pressure-washing decks?

A.James Benney responds: Pressure-washing is a good way to remove dirt and loose paint, but you must use the pressure-washer carefully. Add a mild degreaser such as TSP to the water, and wait a day or two for the walls to dry out before applying paint. Many modern latex paints actually allow moisture to breathe out through their dried film. If using oil paints or stains, let the surface dry for an extra day or two before application. The same holds true for decks.

Remember that it is easy to do a lot of damage in a hurry with a pressure-washer, especially to wood siding and decks. Improperly handled, a pressure-washer can gouge soft wood, drive water into the interior of buildings (staining walls and ceilings), break glass windows and light fixtures, and injure the operator or someone nearby.

When pressure-washing wood siding that has peeling paint, it is important to let the entire surface dry and then scrape by hand. This is because the moisture will also start to lift the paint around the paint that was peeling. You don’t want to paint over this compromised surface, because it will be the first thing to fail.

In California, all deck stains and sealers have been reformulated to meet tough new pollution standards, and this has affected surface preparation. Many manufacturers now recommend that decks be carefully pressure-washed in order to remove all of the previous finish and to open up the pores of the wood to accept the new sealer. Then look forward to repeating the process every couple of years.

James Benney is a member of PaintCraft Associates, a guild of finishing experts in the San Francisco Bay area.