Accumulation of dirt, dust particles and/or other debris on the paint film; may resemble fungus or algae.
POSSIBLE CAUSES:
Use of a low quality paint, especially lower grades of satin types.
Air pollution, car exhaust fumes and flying dust collecting on houses.
Poor building designs / detailing
SOLUTION:
Heavier dirt accumulations may require the use of a power washer. While dirt pickup can't be eliminated entirely, superior grade paints typically offer superior dirt pickup resistance and washability.
Also, higher gloss paints are more resistant to dirt pickup than matt paints, which are more porous and can more easily entrap dirt.
Regular wash down will give longevity to the paint film.
For exterior masonry use Sandtex Trade Dirt Repellent (now with Coloured Indicator)