Mildew is probably the most common cause of paint discoloration. Mildew is a form of fungi which is most commonly black, but some species are red or green. It is usually formed if conditions are prone to grow, on walls where air movement is restricted.
Often mildew resembles dirt on the surface of a paint film. A simple test can be made by applying a drop or two of household bleach solution (5% sodium hypochlorite) to the stain. Mildew will usually bleach out in one or two minutes. Stain that does not bleach is probably dirt. Bleach deteriorates upon aging and loses its potency, so it is important to use fresh bleach.
Before painting in warm, damp climates where mildew occurs (as in the Hampton Roads area) any traces of mildew should be removed and thoroughly rinsed. The mildew must be killed or it can grow through the new coat of paint. Use a bristle brush or sponge to scrub the painted surface, along with the following solution, to kill mildew and clean an area for general appearance or for repainting.
• 1/3 cup household detergent *
• 1 quart (5%) sodium hypochlorite (household bleach)
• 3 quarts warm water
* Do not mix bleach with ammonia or with any detergents or cleansers containing ammonia!
When the surface is clean, rinse thoroughly with fresh water from a hose. Avoid splashing the solution on yourself, or on plants or grass, it may have harmful effects. Before the cleaned surface can become contaminated, allow to dry and repaint with the proper acrylic, latex primer and topcoat containing mildewcides.
While quality exterior oil/alkyds and acrylics both utilize mildewcide additives, mildew progresses more readily on oil/alkyds than acrylics.
Bennette Paint incorporates mildewcides as well as proper pigmentation into their first line products to aid in mildew protection, which is a common problem with the high humidity in our area.