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. 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.