FROSTING

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 lighter tints. On masonry, it can be mistaken for efflorescence (see Efflorescence and Mottling).

POSSIBLE CAUSES:

• Forms mostly in protected areas (such as under eaves and on porch ceilings) that do not receive the cleansing action of rain, dew and other moisture.
• Use of dark-colored paints that have been formulated with calcium carbonate extender.
  Application of a dark-colored paint over a paint or primer containing calcium carbonate extender.

SOLUTION:

• Frosting can be a stubborn problem. It often cannot be washed off readily. Moreover, the condition can recur even as a bleed-through 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; rinse, then apply an alkyd-based primer before adding a coat of high quality exterior paint.

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