Accumulation of dirt, dust particles and/or other debris on the paint film; may resemble mildew.
Use of a low quality paint, especially lower grades of satin or semi-gloss.
Soil splashing onto siding.
Air pollution, car exhaust and flying dust collecting on house body and horizontal trim.
Wash off all surface dirt before priming and painting, using a scrub brush and detergent solution, followed by a thorough rinsing with a garden hose. Heavier dirt accumulations may require the use of a power washer. While dirt pick-up can't be eliminated entirely, top quality exterior latex paints typically offer superior dirt pick-up resistance and washability. Also, higher gloss paints are more resistant to dirt pick-up than flat paints, which are more porous and can more easily entrap dirt.