Mildew is a 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 mildewcide.
  • 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 discolored 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, semigloss or gloss finish, depending on the desired appearance.