This problem occurs when the paint film erodes and the resulting residue appears chalky.
What Causes It?Chalk run-down occurs when paint erodes excessively onto another area below (a brick foundation, for example), ruining its appearance. The problem has a number of causes.
First, the use of a lower-quality, highly pigmented paint or an interior paint can cause the paint to erode prematurely. Factory-finished metal siding can also erode to cause this problem.
How to Solve ItFirst, remove as much of the chalk residue as possible by scrubbing any stained areas with a stiff brush and a detergent solution. Then, rinse the surface thoroughly. In cases of severe staining, an acid wash may be necessary. Either way, if the affected area dries to a different color, consider painting it with a quality latex paint. Eroding aluminum siding should be thoroughly cleaned (power washing recommended) before painting with a quality exterior latex paint.

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