In the growing stage, when the paint surface is damp or wet, the fungus is characterized by its threadlike growth. Mildew will usually bleach out in one or two minutes. Stain that does not bleach is probably dirt. It is important to use fresh bleach solution. Bleach deteriorates upon standing and loses its potency.
Some paints are more vulnerable than others to attack by mildew fungi.
Zinc oxide, a common paint pigment in top coats, inhibits the growth of mildew. Titanium dioxide, another common paint pigment, has very little inhibiting effect on mildew. Considering oil-base paints, mildew progresses more readily on exterior flat house paint than on exterior enamel. Paints containing linseed oil are very susceptible to mildew. Of the available water-base paints, acrylic latex is the most mildew resistant.