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Question posted - January 2 to January 5, 2018:

Do you need to wait for the mortar to cure before repainting a brick surface after repointing and power-tool cleaning to remove loosely adhered coating material from a previous paint job? If so, how long, and is there a way to judge when it is safe?

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Selected Answers

From Zenith Czora of Crommelin on January 10, 2018:
AS/NZS2311, the Australian/New Zealand Standard Guide to the painting of buildings, gives some guidance on the drying times of the concrete required for various types of coatings: 1. Precast and cast in situ concrete surface to be coated with latex paint = 8 weeks 2. Filled concrete blocks to be coated with alkali-resistant, solvent-borne = 12* weeks; oleo resinous and alkyd = 16* weeks 3. Concrete blocks (unfilled) using latex paints = 4 weeks. 4. Cement render and stucco using alkali-resistant, solvent-borne = 6 weeks 5. Concrete bricks and mortar using oleo resinous and alkyd = 12* weeks  *The minimum drying period will depend on the thickness of the concrete and should be increased by four weeks for each 25 mm of thickness above 100 mm. Where two sides of the surface are exposed and well ventilated, then the drying time should be increased by two weeks for each 25 mm of thickness above 100 mm. Where concrete has been wet cured or left exposed during rainy periods, this additional time should be added to the recommended curing time. Typically latex-modified cementitious products, the setting and bond of these coatings are unaffected by the presence of moisture, and they are more forgiving than solvent-based coatings. Lime content of the cement accounts for the high alkalinity (high pH) of the mortar/concrete, and this strong alkaline condition can cause problems for coatings. The inherent alkalinity, when combined with moisture, can be chemically destructive to oil-based or alkyd paints. For paints containing drying oils, risk comes from their susceptibility to saponification, the alkaline hydrolysis of the fatty acid esters. The pH of new concrete can also be checked with pH paper to make certain it is not too alkaline for painting (10 or below is typically desirable). Alkali-resistant, water-based primers are suitable for alkaline surfaces such as concrete and masonry, and are used beneath latex or acrylic finish coats. They also inhibit the occurrence of efflorescence. Various methods can be used to determine the moisture content of mortar. Moisture meters or the plastic sheet method will suffice.

From Michael EDISON of Edison Coatings, Inc. on January 3, 2018:
The answer is that it depends. Some coatings are very sensitive to the high pH in fresh mortar and will require longer curing times before they can be applied. Others are relatively insensitive and can be applied much sooner. Though often omitted, proper repointing procedures include a misting regimen for at least 2-3 days after placement, and depending on depth of repointing and porosities of mortar and masonry, it can take several more days for sufficient drying to occur after that. That suggests a minimum of 5-7 days from placement to coating for the least sensitive coatings. As usual, the coating manufacturer's guidelines should be followed.

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Tagged categories: Moisture-cured coatings; Mortars

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