3. Mildew on Exterior Paint John Stauffer, technical director of the Paint Quality Institute in Spring House, Pennsylvania, says you should make sure you're dealing with mildew and not just dirt: Place a few drops of bleach on a suspected stain, wait a few minutes and then rinse. If the treated area loses its color, it's mildew (bleach does not affect dirt). "Mildew can be eliminated by treating the surface with a mixture of one part bleach to three parts water," Stauffer says. "Leave the mixture on the surface for about 20 minutes, then rinse thoroughly." As always, wear adequate hand and eye protection. While it's impossible to keep your walls free of mildew, there are things that you can do to prevent it from returning. When repainting, clean the surface and paint when it's completely dry. Don't paint on a windy day if nearby surfaces are mildewed, because the spores can blow over and infect the fresh paint. Many high-quality paints on the market contain a mildewcide, but you can also buy an additive to help paint resist mildew. Also, use latex paint, which resists mildew better than oil-based paint because it contains fewer nutrients for the mildew to feed upon. Gloss level also plays a role in mildew growth. Glossier paints are less porous, so dirt and mold spores have less to grab hold of. And because darker colors dry faster than lighter colors after it rains, they are less receptive to mildew.