Frosting appears a white, salt-like substance on the paint surface. Frosting can occur on any paint color, but it is less noticeable on white paint or light tints.
What Causes It?On masonry, frosting can be mistaken for efflorescence. Frosting forms mostly in protected areas (such as under eaves and open porch ceilings) that do not receive the cleansing action of rain, dew, and other moisture.
The use of dark-colored paints that have been formulated with calcium carbonate exterior may also cause this. Applying a dark-colored paint over a paint or primer containing calcium carbonate extender can also lead to this problem.
How to Solve ItFrosting can be a stubborn problem. It often cannot be washed off readily. The condition can also recur as a bleed-through even when a new top coat is applied.
In extreme cases, it can interfere with adhesion. The best remedy is to remove the frosting by wirebrushing masonry or sanding wood surfaces. Then rinse and apply an alkyd-based primer before adding a coat of high quality exterior paint.

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