Repainting a Chalking, Efflorescent Concrete House
A two-storey concrete house is being repainted. The existing coating is an 8-year-old gloss enamel. The owner complains of white powder rubbing off on clothes, and there is a band of white crystalline deposit along the base of the ground-floor wall and under the balcony. About 15% of the wall area shows fine hairline cracks, and three palm-sized blisters have opened near a downspout. A moisture meter pressed on the bare concrete under a blistered area reads 18%. The total wall area is 120 m². The owner wants 'one coat of latex, same color, over the old gloss' so the house looks fresh before a family event in three days. Rain is forecast at 60% for the afternoon. The painter has scrapers, a wire brush, sandpaper, and a can of universal undercoat.
1. Diagnose the wall condition. What is causing the powder, the crystals, and the blisters, and what tests would confirm each?
Read the worked answer
The white powder is chalking: the gloss enamel binder has broken down under sunlight and the pigment is no longer held in the film, so it rubs off. The crystalline band is efflorescence: water moving through the concrete dissolves salts, which are deposited on the surface as the water evaporates. The crystals dissolve in water and are usually alkaline, so a pH test or a simple drop of water will show whether they redissolve, distinguishing them from mould or dirt. The blisters near the downspout, plus the 18% moisture reading, point to water entering behind the film, probably from the downspout or a crack, producing vapour pressure that lifts the paint. A moisture meter on concrete is indicative only, not a laboratory result, so the reading should be checked against a dry reference area and the leak source found. Hairline cracks are shrinkage or thermal cracks and must be assessed for width before filling.
2. Set out the preparation sequence and the coating system you would specify, and justify each step.
Read the worked answer
First fix the water source at the downspout; coating over active moisture will fail again. Then wash the wall with detergent and water to remove chalking, rinse thoroughly and let it dry. Dry-brush the efflorescence off rather than washing it back into the wall, and treat the surface only with a product intended for alkaline masonry. Scrape and sand the blistered areas to bare, sound concrete, feathered at the edges, and check whether the surrounding film is still bonded. Fill cracks and any surface imperfections with a suitable patching compound, then sand flush. Because the existing finish is gloss, adhesion is the main risk: the surface must be deglossed by sanding or a bonding primer used, followed by an alkali-resistant masonry primer on bare and repaired areas. A breathable acrylic masonry topcoat is appropriate; a solvent gloss over damp concrete would trap moisture. Two coats, not one, give uniform film thickness.
3. The owner insists on one coat over the old gloss before the event. What is your professional response, and what schedule do you propose?
Read the worked answer
I would explain the consequence in the owner's terms: at 18% moisture and with active efflorescence, a single coat over gloss will blister, peel and chalk again, probably within one rainy season, and the money spent will be wasted. One coat also cannot hide the colour and sheen difference between old gloss enamel and new latex, so the finish will look patchy for the event anyway. I would not simply refuse, but offer a phased plan: carry out the moisture source repair and washing now, let the wall dry to an acceptable level, apply the primer and the first full coat before the event if the weather holds, and apply the second coat after. If rain arrives, work stops; moisture readings are retaken before recoating. This is documented and agreed in writing so the client understands what is guaranteed and what is not. Professionalism means protecting the client from a predictable failure, not just doing what is asked.