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.