Efflorescence and Damp Lime Plaster by the Sea
A 30-year-old coastal house has a 120 m² external lime-plaster wall. White fluffy salts appear up to 1.2 m above ground, paint is blistering, and there are hairline cracks. A moisture meter gives 18–22% in the lower band and 11–13% higher up. The surface pH is 9.5. Previous finish is distemper over a chalky, friable layer; some plaster sounds hollow over about 3 m². The owner wants a quick acrylic emulsion repaint before a family function in ten days. A helper proposes washing the salts off with water, applying a coat of cement paint, and painting over the damp patches. The wall faces the sea, and rainwater pipes discharge onto the plinth. The painter has a wire brush, a scraper, ordinary filler, and one drum of alkali-resistant primer.
1. Diagnose the likely cause of the white salts and evaluate the helper's proposed method.
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The white fluffy salts are efflorescence, a symptom of soluble salts being carried to the surface by moisture and crystallising as the wall dries. The moisture readings are much higher in the lower band, and the rainwater pipes discharge onto the plinth, so the likely cause is water ingress at the base, not a paint fault. The hollow plaster over about 3 m² suggests loss of bond, probably from long-term damp.
The helper's proposal is not acceptable. Washing the salts with water adds moisture and can drive salts back into the plaster. Cement paint is not a waterproofing cure and may trap moisture or be incompatible with the existing distemper and later acrylic system. Painting over damp patches will almost certainly cause blistering and recurrence. The correct approach is to find and control the water source first: redirect rainwater pipes, check the plinth and drainage, and allow the wall to dry. Only then should friable distemper and loose plaster be removed, salts dry-brushed off, hollow areas cut back and repaired with a compatible mortar, and a test patch primed and painted. The owner should be warned that a ten-day decorative repaint cannot reliably solve an active damp problem.
2. Set out a practical preparation and coating sequence, including what must be checked before the finish coats.
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A defensible sequence is: 1) correct the rainwater discharge and any obvious external water path; 2) allow the wall to dry, monitoring moisture rather than guessing; 3) dry-brush salts from the surface and remove friable distemper with scrapers, not by washing; 4) cut back and repair hollow or cracked plaster with a compatible lime or renovating mortar, allowing proper cure; 5) treat any organic growth with a suitable fungicidal wash if present, then rinse and dry; 6) abrade and dust off; 7) apply an alkali-resistant primer/sealer compatible with the acrylic emulsion, especially given pH 9.5; 8) apply a test patch of the full system and check adhesion, drying and salt recurrence; 9) apply the specified finish coats.
Before finish coats, the moisture content should be within the coating manufacturer's limit, the pH should be acceptable for the primer, and the test patch should show no blistering, lifting or new salt. The painter should record readings and obtain client acceptance that damp-source repairs are outside the painting scope if they are not included. If moisture remains high, the finish should be postponed rather than sealed over.