Crusty, white salt deposits, leached from mortar or masonry as water passes through is known as mottling.
Generally mottling occurs when you fail to adequately prepare a surface by removing all previous efflorescence. This leads to moisture escaping through the exterior masonry from the outside leaving the unsightly, white salt deposits.
Paint application substrate must return well under 10% moisture content; any moisture ingress must be isolated. Eliminate the source of the excess moisture by repairing the roof, cleaning out gutters and downspouts, and sealing any masonry cracks with a high quality, water-based all-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. Dulux Weathershield Matt is recommended.
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