Efflorescence or mottling appears as crusty, white salt deposits, leached from mortar or masonry as water passes through.
What Causes It?Efflorescence or mottling is caused by failing to adequately to prepare the surface by removing all previous efflorescence or when excess moisture escapes through the exterior masonry walls from the inside.
How to Solve itIf excess moisture is the cause, eliminate the source by repairing the roof, cleaning out gutters and downspouts, and sealing any cracks in the masonry with a high-quality, water-based all-acrylic or a siliconized acrylic caulk.
If moist air is originating inside the building, consider installing vents or exhaust fans, especially in kitchen, bathroom, and laundry areas.
Remove the efflorescence and all other loose material with a wire brush, power brush, or power washer; then thoroughly rinse the surface. Apply a quality water-based or solvent-based masonry sealer and allow it to dry completely; then apply a coat of top-quality exterior house paint, masonry paint, or elastomeric wall coating.

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