Efflorescence and Paint Blistering on External Plaster
An external brick wall is plastered with a 1:6 cement–sand mortar in 12 mm thickness. The plaster is applied three days after the wall is built, and the wall surface is not wetted before plastering. The plaster is cured for two days, then painted with one coat of primer and two coats of acrylic emulsion after another two days. Within a week of the first rain, white powdery patches appear near the plinth, and paint blisters and peels in patches up to 300 mm across. The client blames the paint. The contractor blames the bricks. The PSEB trainee is asked to inspect and recommend a repair that will last. The bricks are local burnt-clay bricks; some pallets were stored on wet ground. There is no DPC visible at the plinth.
1. What is the likely diagnosis, and how do the observed symptoms support it?
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The pattern points to moisture and soluble salts, not a simple paint failure. White powdery deposits are efflorescence: water moves through the brick or mortar, dissolves salts, and deposits them at the surface as it evaporates. Paint blisters and peels because moisture is trapped behind a low-permeability acrylic film; the salt crystals also push the film off. Plastering three days after the wall, not wetting the wall, and curing for only two days leave the plaster damp and weak. Painting after only two more days seals that moisture in. Bricks stored on wet ground can add salts. The plinth area and missing DPC suggest rising damp or splash-back as the water source.
2. Outline the repair sequence.
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Find and cut off the water source first: check DPC, plinth protection, drainage, leaking pipes and ground level. Remove blistered paint and loose plaster back to sound material; dry-brush efflorescence off the surface and do not wash it back into the wall. Repair plaster with a compatible mix, cure it for at least 7 days, and let it dry thoroughly. Check dryness with a plastic sheet taped over the wall for 24 hours; if condensation appears, wait. Apply an alkali-resistant primer and a breathable exterior paint system, or use a salt-resistant coating as specified. Do not simply paint over the patches.
3. What should have been done differently during construction and painting?
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Wet the bricks and dampen the wall before plastering so the mortar does not dry out too fast. Use a measured 1:6 mix and apply the specified thickness. Cure the plaster for at least 7 days, not two. Allow the plaster to dry before painting; do not apply acrylic emulsion to a damp substrate. Keep bricks off wet ground and protect them from rain. Detail the DPC and plinth drainage so rising moisture is cut off. These changes address the source and the moisture balance, not just the appearance.