Why Paint Won't Dry on Concrete

Curing Concrete

• Concrete is a mixture of cement, crushed rocks of various types of minerals, sand and water. As wet concrete dries, the moisture evaporates leaving behind a strong, dense material. Although it may appear to be completely dry, concrete generally requires a minimum of 28 days to fully cure. The length of time can vary based on the concrete mixture and the ambient temperature. For best results, wait at least one month before attempting to paint newly poured concrete.

Cleaning Concrete

• Paint will not adhere to a concrete surface if it is coated with grease and grime. Scrape off any debris, moss and efflorescence and clean with a commercial concrete cleaner containing a degreaser. Alternatively, scrub the surface with a soapy mixture of hot water and detergent or trisodium phosphate, widely known as TSP. Rinse well with clear water and allow the concrete to thoroughly dry.

Moisture Problems

• While it may seem impenetrable, concrete is a porous material. Exposure to the elements or plumbing leaks can allow water to seep through unsealed or improperly sealed concrete. Check for possible sources of leaks and make repairs before applying a penetrating sealer. To test for moisture, tape a 2-foot-by-2-foot square of plastic sheeting on the concrete. Seal the edges well and wait 48 hours before peeling off the plastic. If there is condensation on the concrete or underside of the plastic, the moisture content may be too high for the paint to adhere.

Surface Preparation

• After the concrete surface is washed, dried and sealed, check for cracks and flaws and patch with epoxy or concrete fillers if necessary. Sand glossy areas until dull for better paint adhesion. Smooth concrete surfaces may need to be roughened with sandpaper or an etching solution. Apply a high-quality masonry primer according to the manufacturer's directions before applying concrete paint.