What causes white powdery deposits on masonry surfaces, and how can I fix them?

The deposits, known as efflorescence, are caused by moisture in the masonry. The moisture dissolves salts in the masonry — the mortar, block, concrete, etc. — and is drawn to the surface by the heat of the sun. The water evaporates, leaving behind white deposits that must be removed before painting. After you find and fix the source of the moisture, remove the efflorescence with a wire brush and coat the surface with a masonry primer and topcoat.