Dirt, dust particles, or other debris can accumulate on the paint film, which may resemble mildew.
What Causes It?Dirt can accumulate on lower-quality paints, especially lower grades of satin or semi-gloss.
Usually, dirt pickup is caused by soil splashing onto siding or results from air pollution, car exhaust, and flying dust collecting on the house body and horizontal trim.
How to Solve ItWash off all surface dirt before priming and painting. If unsure whether the problem is dirt or mildew, conduct a simple spot-test (see Mildew). Clean off dirt with a scrub brush and detergent solution, followed by a thorough rinsing with a garden hose. Heavier dirt accumulation may require the use of a power washer.
While dirt pickup can’t be eliminated entirely, top-quality exterior latex paints typically offer superior dirt pickup resistance and washability. Also, higher-gloss paints are more resistant to dirt pickup than flat paints, which are more porous and can easily entrap dirt.

Note: Images provided by The Rohm & Hass Paint Quality Institute