Chalking:

Chalking, the formation of fine powder on the surface of the paint film during weathering, which can cause color fading. Although some degree of chalking is normal, desirable way for a paint film to wear, excessive film erosion can result in heavy chalking.

Possible Cause:
Use of a low-grade, highly pigmented paint
Use of an interior paint for an outdoor application
Solution:First, remove as much of the chalk residual as possible, use a stiff bristle brush (or wire brush on masonry) and then rinse thoroughly with a garden house; or use power washing equipment. Check for any remaining chalk by running a hand over the surface after it dries If noticeable chalk is still present, apply a quality oil-based or acrylic latex primer (or comparable sealer for masonry), then repaint with a quality exterior coating; if little or no chalk remains and the old paint is sound, no priming is necessary.

Note: Pictures provided by the Rohm + Hass Paint Quality Institute.