Frosting

A white, salt-like substance on the paint surface. Frosting can occur on any paint colour, but it is less noticeable on white paint or light tints. On masonry, it can be mistaken for efflorescence (see Efflorescence/mottling).

Possible causes:

• Forms mostly in protected areas (such as under eaves and on open porch ceilings) that do not receive the cleansing action of rain, dew and other moisture.
• Use of dark-coloured paints that have been formulated with calcium carbonate extender.
• Application of a dark-coloured 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 wire brushing masonry or sanding wood surfaces; rinse, then apply an alkyd-based primer before adding a coat of high quality exterior paint.