The Problem: A fungus (mold) that grows on exterior surfaces, as well as interior walls (bathrooms and basements) that are subjected to humidity and poorly ventilated. If left untreated, mildew will eat away the existing paint film and cause failure in affected areas.
Causes: Exterior surfaces that tend to stay damp and receive little or no direct sunlight, as well as interior walls exposed to high humidity and other forms of moisture. Failing to prime bare wood surfaces prior to applying the topcoat, and painting over a surface where mildew is already present.
How-to-fix: The first step is to test for mildew by applying a few drops of bleach to discolored areas. If the dark color bleaches out, it’s likely mildew. Remove the mildew by applying a commercially prepared solution or diluted house-hold bleach. Rinse the treated surface thoroughly. When removing mildew from exterior surfaces, make certain to protect plants and shrubs with drop cloths. Finally, allow the surface to completely dry before applying primer and paint.