Older homes or homes with basements dug below the frost line often have moisture problems with their concrete floors. When it comes to painting, dampness is often the enemy of the finished product, as the paint can peel and bubble away from the concrete if it is damp. Fortunately, there is a simple way to paint damp concrete while preserving the beauty of the finished product.
Determine why your concrete floor is damp. If the dampness is an ongoing problem, you may need to undertake structural repairs, or repair any leaking appliances. If the dampness is not an ongoing problem, wait for the concrete to dry thoroughly before attempting to paint it. To test whether your concrete floor is still damp, cut a 3 inch by 3 inch square of plastic and tape all sides to the floor. Wait 24 hours and look under the plastic. If the plastic and the floor below it are dry, then proceed. If not, try putting a dehumidifier or desiccant material in the area to reduce the amount of ambient moisture in the room and repeat the test again.
If your concrete floor is old or stained, clean it thoroughly with a solution of trisodium phosphate before sealing it. Mix the trisodium phosphate with water according to the package directions, and use the push broom to spread it over the floor. When the trisodium phosphate dries, sweep or vacuum up any residue.
Seal the concrete. Use one of the floor paint rollers to spread a thin layer of sealant over the concrete floor. Wait at least 24 (and preferably 48-72) hours for the sealant to dry as much as possible. This will prevent excess moisture from seeping up through the already damp floor and ruining the paint finish.
Prime the concrete floor with damp concrete primer. Make sure that the product is specifically made for damp concrete floors. Use the floor paint roller to spread a thin layer of primer over the entire floor. Wait 24 hours and give the floor a second coat. Wait 48 hours before proceeding.
Paint the concrete floor using concrete floor paint. Make sure that the paint is specifically made for floors, as normal latex paint will produce a slippery finish, especially on floors prone to dampness. Use a third floor paint roller to spread a thin coat of floor paint over the concrete floor. Wait 24 hours and apply a second coat.