This occurs when chalk from excessively eroded paint washes down onto another area below it (a brick foundation, for example), ruining its appearance (see Chalking).
- Using a lower-quality, highly pigmented paint.
- Using an interior paint for an outdoor application.
Remove as much of the chalk residue as possible (see Chalking). Scrub 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 colour, consider repainting it with a quality water-based paint.