CAUSE: Mildew is NOT caused by paint. It is a fungus that grows on many surfaces including paint. Mildew discoloration is prevalent in warm, moist exposures, but can occur in most climates. Mold and mildew spores are microscopic particles transported by the wind. Spores can remain dormant on a surface until conditions are favorable for growth. Mildew-contaminated surfaces in neighborhoods can supply spores for contamination of other homes in the area. Mildew usually looks similar to dirt accumulation. However, on some surfaces it may appear as dark spots, or be concentrated in clusters. Under magnification a web like pattern is visible. Mildew is usually black or gray in color, but can also be dull red or yellow. To determine if mildew is present, drop a small amount of bleach on discolored surface. If mildew is present, bleach will destroy it and lighten the spots. If discoloration is simply dirt, bleach will have no effect.

SOLUTION: Mildew MUST be removed before painting. Painting over mildew guarantees its return. Mildew will grow through paint films rapidly, and correction will then require removal of the coating! All mildew-stained surfaces should be washed with a solution of 1 or 2 cups of household bleach per gallon of warm water shortly before painting. Commercial mixtures for washing the surface are also available. Repeated applications may be necessary to completely remove discoloration. Repaint as soon as the surface is dry with a mildew-resistant paint. Multiple coats of quality products resist mildew growth longer than single coats. While there is no absolute cure for mildew, quality products will contain mildew resisting properties that can minimize its growth. Periodic examination and treatment of painted surfaces, in sound condition, with mildewcide wash will restore a freshly applied appearance to the film. Due to their composition, latex products are less likely to grow mildew than oil-based products.

Above article reprinted by the kind permission of Benjamin Moore Paints.