Black, gray or brown areas of fungus growth 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 vulnerable).
Use of a lower quality paint, which may have an insufficient amount of mildewicide.
Failure to prime bare wood before painting.
Painting over a substrate or coating on which mildew has not been removed.
Test to distinguish mildew from dirt by applying a few drops of household bleach to the discoloured area; if it disappears, it is probably mildew. Treat the mildew by applying a mixture of water and bleach, 3:1, and leave on for 20 minutes, applying more as it dries. Wear goggles and rubber gloves. Then scrub and rinse the area. Apply an exterior latex primer, then a top-of- the-line exterior latex paint in flat, satin, semi-gloss or gloss finish, depending on the desired appearance.