Sand when dry. Mildew is NOT caused by paint. It is a fungus that grows on many surfaces including paint. Mildew discoloration is prevalent in warm, moist exposures, but can occur in most climates. Mold and mildew spores are microscopic particles transported by the wind.
Spores can remain dormant on a surface until conditions are favorable for growth.
Mildew-contaminated surfaces in neighborhoods can supply spores for contamination of other homes in the area. Mildew usually looks similar to dirt accumulation. However, on some surfaces it may appear as dark spots, or be concentrated in clusters. Under magnification a web like pattern is visible. Mildew is usually black or gray in color, but can also be dull red or yellow.