Mildew

Mildew
Black, gray, green, or brown spots on any surface. Paint does not cause mildew.

POSSIBLE CAUSE

Mildew forms interior and exterior surfaces that tend to be damp or receive little or no direct sunlight.
Painting over a substrate (e.g. the surface that was painted) containing mildew.

SOLUTION

Test for mildew by applying a few drops of household bleach to the area, if it is bleached away, the discoloration is probably mildew; if it does not disappear, it is probably dirt.
Before painting, remove mildew by washing the surface with appropriate Concrobium, Krud Kutter, or Moldex cleaners
To protect against mildew, use a top-quality paint, and clean when necessary. Install an exhaust fan in high-moisture areas.
Use a product that contains agents that inhibit the growth of mildew on the surface of paint film.
If mildew is the only problem, painting may not be necessary after cleaning the surface.

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