Q: The siding on the northern exterior of our house has a constant mildew problem. I have installed vents on the gable ends of the roof, washed the siding and repainted. After all this work, the mildew is reappearing. Before I repaint again, do you have any suggestions on how to prevent recurrence of this problem?

A: Repainting is not the answer until the mildew is removed. Otherwise the remaining mildew will continue to grow through the new paint. After the mildew growth has been well established on a painted surface, it should be removed with a solution that effectively kills the spores.

Use one of the following formulas to thoroughly scrub the siding: Mix one-third cup of powdered detergent, one quart chlorine bleach and three quarts warm water; or mix two-thirds cup of trisodium phosphate, one-third cup powdered floor cleaner and one quart of chlorine bleach with three quarts warm water; or try mixing household bleach with M-I Housewash.

Use a stiff-bristle brush and scrub the solution onto the surface. Continue scrubbing until the mold has been removed and clean paint is exposed. If the existing paint is glossy, first sand it or apply a deglossing product.

Once the surface is dry, apply a new coat of paint as soon as possible. Use an acrylic-latex paint containing a mildewcide, or you may want to try Zinsser's Perma-White, Mildew-Proof Exterior Paint. Zinsser's, aN acrylic paint, comes with a mildew-proof, five-year warranty. While available in white only, it can be tinted to off-white or pastel shades. The pain is recommended for use on wood, stucco, concrete, stone, brick, vinyl siding or aluminum siding.

If mildew returns, you may have to remove the paint down to the bare wood, thoroughly wash the siding with one of the recommended solutions and then repaint.

Mildew is caused by airborne spores that are present nearly everywhere in warmer, more humid climates. Prolonged warm weather and relatively high humidity favor mildew growth on unprotected organic material, especially when the surface is shaded from the direct rays of the sun and is located where there is inadequate air circulation. A mildewcide paint is formulated to resist the growth of mildew under climatic conditions that favor it. It is important that there is no appreciable delay between scrubbing the mildew off and repainting with a mildew-resistant paint. Simply washing off existing mildew does not proof the surface against a new attack. Trimming shade trees in the area to allow more direct sunlight on the house will also help.

Write to Here's How, Copley News Service, P.O. Box 120190, San Diego, Calif. 92112-0190, or e-mail copleysd@copleynews.

com. Only questions of general interest can be answered.