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: Mildew is a fungus found virtually everywhere; all it needs in order to flourish is oxygen, a food source, moisture, and temperatures between 40°F and 90°F.

As a siding stain degrades — due to UV exposure and other environmental factors — the mildewcide it contains breaks down. And if the climate is right (warm and humid is ideal), chalked siding offers the perfect surface for mildew growth, with dirt that's accumulated over the years providing an additional food source.

Oil-based stains are more prone to mildew growth than latex stains, but both types need good mildewcides. These typically are effective for only two or three years; if mildew reappears after just six months, most likely the old surface was not adequately cleaned of dirt, chalk, and old mildew before restaining. If mildew is present when you restain, you actually help it by providing food and moisture for the spores; furthermore, the stain could prevent bleach solutions from reaching the source of the mildew.

To remove mildew before restaining and to prevent it from reappearing, use a solution of one quart liquid household bleach containing 5 percent sodium hypochlorite and three quarts warm water; or use a commercial mildew cleaner, such as those containing sodium percarbonate. Also, be sure to scrub the surface gently with a sponge mop or soft-bristle brush after applying the bleach or cleaner (always be careful to protect the skin and eyes during application). After 15 to 20 minutes, thoroughly rinse with water to remove any remaining cleaner or bleach solution, and let the cleaned siding dry properly before restaining.