Black, gray or brown spots or areas on the surface of paint or caulk.
Forms most often on areas that tend to be damp, or receive little or no direct sunlight (e.g., bathrooms, kitchens and laundry rooms).
Use of an alkyd or oil-based paint, or lower quality latex paint.
Failure to prime bare wood surface before applying the paint.
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 area; if it is bleached away, the discolorant is probably mildew. Remove all mildew from the surface by scrubbing with a diluted household bleach solution (one part bleach, three parts water), while wearing rubber gloves and eye protection. Rinse thoroughly. To protect against mildew, use a top quality latex paint, and clean when necessary with bleach/detergent solution. Consider installing an exhaust fan in high moisture areas.
(image courtesy of The Rohm and Haas Paint Quality Institute)