To clean mildew off painted walls, wear rubber gloves, then mix 3/4 cup bleach with 1 gallon of water, and apply to the surface with a soft brush. A nuisance and a potential hazard to the health of the whole family, mold forms quickly on damp walls, especially in areas of poor ventilation. According to Good Housekeeping, it is important to clean the surface at the first sign of mold growth.
After applying the solution, let it stand for 15 minutes before rinsing the area again with clean water. It is important to thoroughly dry the area with a fan for about half an hour to make sure the surface is dry. If the stain reappears after a short time, the mold may be infesting the drywall, and it may have to be replaced. Once the mildew has been removed, it may be necessary to repaint the cleaned wall surface with a regular latex paint.
According to The Family Handyman, the key to stopping mold is to control dampness. Adding a dehumidifier in the home helps keep walls dry and limits the conditions that are favorable for mold development. Also, many new paints contain a chemical called mildewcide that effectively controls surface mold in damp rooms.