Gray, fan-shaped areas spread below these spots as spores from the original colony multiply and are washed down the surface. Severely infested areas may appear uniformly gray or black. Mildew spots will disappear within one or two minutes. Dark spots which are not removed by bleach may be dirt, extractive or iron stains. To remove a mild case of mildew, scrub the surface with a mild cleanser or detergent.
High concentrations cause a discoloration referred to as extractive bleeding.
Extractive bleeding appears in several forms. Some boards, high in extractive content, may turn very dark if unprotected by finishes and exposed to moisture. Extractives may bleed through paints if stain-blocking oil or alkyd primers are not used. In other cases, extractives may discolor the face of paints or other finishes if they are allowed to drip over them. Condensation on the unprimed back side or wicking under the lap from the face of bevel siding are common causes of this.