If you have mildew on the exterior of your home, don't paint over it until you have resolved the source of the problem with mildew removal procedures. If you fail to remove mildew—and other foreign particles such as algae, dirt or soot—from your home's surface, your subsequent paint job will deteriorate prematurely. In fact, severe mildew on paint surfaces and dirt contamination can lead to peeling within a year or two of application.
However, if you do a thorough job of cleaning the dirt and mildew on the paint of your home, you may find that the home looks fresh and attractive even without painting.
How did mildew get on your house in the first place? Mildew is the visible part of very tiny plant life called fungi. Mildew can grow on paint because, first, paint may contain organic materials that nourish the plant life and, second, organic material floating in the air falls on the painted surface to become food for mildew.
Mildew grows best in temperatures between 70 and 90 degrees F. Below freezing, it become dormant. Though mildew is found in every state and province, it is much more plentiful in warm, humid climates such as the Gulf region. If you live in a region where mildew is a great problem, you may want to consider using a paint containing mildewcide or using a mildewcide additive.
Mildew grows more quickly on exterior alkyd flat paint than on semi-gloss or gloss enamels. It also grows quickly on coatings containing linseed oil. Latex paints as a whole are very susceptible to mildew attack, though acrylic paint seems to be the most mold-resistant of the latex paints.
Dirt, meanwhile, is also an enemy to any paint job. Dirt, soot or soil is easily distinguishable from mildew. Just apply a small amount of household bleach to the surface. The bleach will whiten fungi or algae but does not change the color of dirt.
So now that you know what mildew is, what do you do if you find it on your home's exterior? Though household bleach can be used to remove mildew, commercially prepared washes both kill mildew and cause mildew and dirt to release from the surface with little or no scrubbing. However, rough-sawn wood and stucco may need aggressive scrubbing to ensure complete cleaning. If your exterior surfaces smell musty, a combination of vinegar, soap, and hot water can aid in mildew odor removal.
Never mix bleach with ammonia or any detergents or cleaners containing ammonia, however. Bleach and ammonia in combination can be lethal. People have actually died from breathing the fumes from such a mixture. Many household cleaners contain ammonia.
Once you have removed the mildew and dirt from your home, you may decide that it looks good enough to forgo a paint job this year. But if a paint job is still in order, visit your local paint store for products and advice.