What Causes Powdery White Blotches or Mottling in Paint on a Stucco Exterior Wall?
Our photo (left) shows white blooming efflorescence and mottling appearing on painted stucco after less than a year of application of paint onto a new stucco exterior wall in the Southwestern U.S. See PAINT on STUCCCO, FAILURES for details of paint failure diagnosis on stucco surfaces. We also discuss efflorescence as a type of paint problem at PAINT FAILURE DICTIONARY. Painting over cracks where moisture has not fully evaporated or painting over a new stucco wall too soon and where the wall pH or alkalinity remains too high (over 11) can lead to both cosmetic problems as well as early wear or failure of the painted coating. The application and curing procedure used for stucco, in turn affect the wall pH – it needs to be tested by the painter before the paint job begins.
