Black, grey or brown spots or areas on the surface of the paint.


  • Forms most often on areas that tend to be damp, or receive little or no direct sunlight, e.g. bathrooms, laundry rooms
  • Use of an oil based paint, or low quality waterbased paint
  • Failure to prime the bare wood surface before applying the topcoat
  • 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 discoloration is probably mildew. Remove all mildew from the surface by scrubbing with a diluted household bleach solution (1 part bleach to 3 parts water), while wearing rubber gloves and eye protection. Rinse thoroughly. To protect against mildew, use a high quality waterbased paint and clean when necessary with the above bleach solution or neutral detergent. Consider installing an exhaust fan in high moisture areas.

Source: The Paint Quality Institute

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