A brown, gray, or black stain on siding or trim may be simple grime, or it may be mildew. To test whether a stain is grime or mildew, try washing the stain away with water and a detergent like Spic and Span. If the stain doesn't wash away with water and detergent, it's probably mildew.
Because mildew is a fungus growth, it thrives on moisture and dirt, so keep the siding open to the sun and air. Don't store firewood or other materials close to the house. Prune tree branches that shade the house. If you have recurring mildew problems, power-wash the house's exterior once a year and apply a mildewcide solution every two to three years.
Handle bleach and mildewcide solutions with care. Read label warnings, and always wear protective clothing, especially neoprene gloves and goggles. Protect shrubs and other plantings by watering them down well and covering them before you power-wash the house. Water them well again after.
Some exterior paints, such as those used on aluminum siding, intentionally chalk for self-cleaning. Wipe the surface with the palm of your hand. If the paint color comes off on your hand, the paint is chalking. Scrub off the chalk with a strong solution of water and Spic and Span or TSP detergent (or a nonphosphate TSP alternative). You can also use a commercial wood cleaner to remove chalked paint. Water pressure alone doesn't do the trick.As you scrub the surface, work from the bottom up to avoid streaking. Rinse frequently with clear water, and allow the surface to dry before painting.
Efflorescence, mold and algae, lichen, and stains from chalking paint or rusting metal are problems that you should attend to before painting masonry, which includes surfaces such as stucco, brick, and concrete block.First, eliminate the sources of the problems. Overhanging trees can cause mold and algae; a deteriorating chimney cap can allow water to penetrate behind the brick and cause efflorescence. Rusting may result from the use of various metals reinforcing materials in masonry construction, such as wire mesh or steel structural materials over windows and doors. Even if the metal isn't visible, the rust stains may bleed through to the surface. If the paint on the siding above masonry is chalking, it washes down onto the masonry and stains the surface.
In most cases, you need to scrub the surface with a chemical cleaner. For efflorescence and other stubborn stains, mix 1 cup of 10 percent muriatic acid in 1 gallon of water. (Muriatic acid is available at your local hardware store.) Heavy stains may require up to a 1:1 solution, but the risk of damage to the surface increases. Scrub the surface with a stiff brush and rinse thoroughly with water. Strong acid solutions must be neutralized with a 1:10 ammonia-water solution to stop the etching.
Add the acid to the water or it will spatter. Because the acid vapors can burn your lungs, keep your distance from the acid by using a long-handled brush to scrub the surface. Wear protective gear, including a respirator, heavy neoprene gloves, and splash-proof goggles, and read the label for additional handling instructions.