Mould or mildew is the formation of brown, black or grey spots or blotches on the surface of paint or caulk due to the presence of fungi that feed on the paint film and other organic matter.

• Damp areas that receive little or no direct sunlight (thus, such areas as walls with a southern exposure and the underside of eaves are particularly vulnerable)
• Use of a lower quality paint with an insufficient amount of mildewcide
• Painting over a substrate or coating on which mildew is present
• Failure to prime bare wood before painting
  
  

• First, make sure that the discoloration really is mildew, and not just dirt. Apply a few drops of household bleach to it, wait five minutes, then rinse. If the dark colour disappears, the discoloration is probably mildew. If it does not disappear the discolouration may just be dirt and dust.
• Remove all mildew by scrubbing with a diluted household bleach solution (one part bleach, three parts water). Be sure to take the proper safety precautions - wear rubber gloves and eye protection. Leave the bleach on the surface for several minutes, then rinse.
• Next, thoroughly clean the surface with a detergent solution to remove dirt and other organic material on which mildew can feed. This can be done by hand or with a pressure washer.
• Rinse again. When the surface is dry, apply one or two coats of an ultra premium quality exterior acrylic paint - this type of paint contains extra mildewcide and will resist mildew better than oil-based or lower quality water-based paints.