Question: I have a concrete porch that’s been painted several times. But the paint bubbles and peels off, leaving a mess. Each of the painters has said that this happens to concrete when you paint it over the winter. I’m not sure that they are telling me the truth. How do I get the porch painted in the correct manner?
Answer: A variety of issues could be at play here. First, you should determine whether moisture is coming up through the concrete from below. Tape a piece of relatively thick, clear plastic, such as a three-inch square cut from a reclosable plastic bag, to an area with no paint. If moisture droplets appear over the next day, you’ll need to strip off all the paint and switch to a coating that resists hydrostatic pressure. Rust-Oleum’s WaterTite-LV Waterproofing Concrete Masonry Paint is one example. Or you can coat the stripped surface with a concrete-resurfacing material, such as Quikrete’s Concrete Resurfacer, which can be tinted.
If no moisture beads form, a suitable paint should stick, provided it is properly applied. You or your painter needs to scrape off all loose paint and scuff up what remains with 100-grit sandpaper. Wipe off the sanding dust with a damp rag, then clean the concrete with a cleaner-degreaser, rinse and allow to dry. Coat all areas of bare concrete with a suitable primer, such as Behr Concrete & Masonry Bonding Primer. That needs to cure at least eight hours. Then you can paint with a product designed for exterior concrete, such as Behr Premium Plus Porch & Patio Floor Paint.
All of the above is fairly standard advice, so your painters may have done that. But they might not have realized some of the details that Mariah Sanchez, senior technical services representative for Behr, says are also important. First, when the paint label tells you what the temperature must be (often 50 degrees for water-based paints), it’s referring to the surface temperature of the concrete. That is typically 10 to 15 degrees cooler than the air temperature. So for most water-based paints, the air temperature needs to be at least 60 degrees.
Second, you can’t spread a thicker coat than the label recommends or rush the time needed between coats. Being dry to the touch isn’t what counts; paint molecules next to the concrete also need time to cure. For the Behr paint, that means waiting 24 to 48 hours between coats, or even longer if the weather is cool or the relative humidity is above 50 percent. Recoating too soon essentially turns two thin layers, normally a good thing, into one thick layer that’s prone to peeling.
Finally, you need to make sure the paint isn’t hit by rain for at least a week. For all of the above reasons, repainting a concrete porch isn’t usually a good wintertime project.
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g Read Jeanne Huber’s roundup of home-improvement tasks you should tackle in June, such as cleaning the deck, at washingtonpost.com/home.