Black, gray or brown areas on the surface of paint or caulk.
> Forms most often on areas that tend to be damp, and
receive little or no direct sunlight (walls with a northerly
exposure and the underside of eaves are particularly
> Use of a lower quality paint, which may have an insufficient amount of mildewcide.
> Failure to prime bare wood before painting.
> Painting over a substrate or coating on which mildew has not been removed.
Test for mildew by applying a few drops of household bleach to the discolored area; if it disappears, it is probably mildew. Remove all mildew from surface by scrubbing with a diluted household bleach solution (one part bleach, three parts water); wear rubber gloves and eye protection. Power washing is also an option. Rinse thoroughly, prime any bare wood then apply one or two coats of top quality exterior paint, which typically contains mildewcide.
Source: Paint Quality Institute