The washing down of chalk from an excessively eroding paint onto another area below (a brick foundation, for example), ruining its appearance (see Chalking).
Possible Causes:
Use of a lower quality, highly pigmented paint.
Use of an interior paint for an outdoor application.
Erosion of factory-finished metal siding.
Solution
Remove as much of the chalk residue as possible (see Chalking). Scrub any stained areas with a stiff brush, using a detergent solution; rinse 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.
(image courtesy of The Rohm and Haas Paint Quality Institute)