Moisture is likely reason for paint lifting off floor

Q The concrete floor in our 1924 house basement has been painted several times. Now, the paint is lifting in small and large spots, sometimes taking concrete with it. How does one best deal with this?

A Although there are a couple of reasons for paint not adhering to a concrete basement floor, typically it's a moisture problem.

What happens is soil moisture wicks through the concrete, picking up alkali as it travels. At the surface (the basement floor), the moisture evaporates, leaving alkali crystals behind. The alkali pushes the paint film away from the surface, resulting in the peeling.

In most newer homes, there's a moisture barrier under the slab, so moisture migration isn't a problem. But that's not the case with older homes like yours.

If peeling paint is a recent problem, it might be that moisture conditions have changed around your home. The addition of a sprinkler system or new landscaping can alter drainage patterns. In addition, hard surfaces around the home (sidewalks, driveways) can shift over time, altering drainage patterns. Basically, if rain and snowmelt runoff flows toward the foundation instead of away from it, the soil becomes charged with moisture, which is readily sopped up by the foundation's concrete slab and walls.

The first step is to check that the ground around your home, which also can settle over time, actually slopes away from the foundation. (Be sure it's soil that's sloped, not just rock or wood mulch.) Overgrown plants near the foundation can also contribute to the problem. Be sure the gutters and downspouts discharge water at least 10 feet from the foundation. Fix any hard surfaces that slope the wrong way.

When you've done as much as possible to eliminate moisture migration, you can prepare to repaint. The experts at Dutch Boy paints recommend that you remove all loose paint by scraping or using a chemical stripper. Scrub with hot water and a strong detergent or degreaser. Rinse thoroughly and allow the surface to dry for several days. Once the floor is clean and dry, acid-etch with phosphoric or citric acid, following label directions. (Check home, hardware or paint stores for these products.) When dry, paint again.

Remember, there's a simple moisture migration test for concrete. Cut open a plastic garbage bag (or several). Secure to the floor by taping around the perimeter, creating a tight seal. After several days or a week, lift and check for moisture. Ideally, there should be none.

Send your questions to Fixit in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-9033, or e-mail fixit@startribune.com. Past columns are available at www.startribune.com/fixit.