Prepping painted concrete.
My outdoor porch has a concrete floor and steps. It was painted, probably 50 years ago, with brick-red paint, and repainted more than once with latex paint, which did not adhere well. The red paint has worn in places, but appears extremely durable. The Rust-Oleum concrete paint stripper and 3000 psi power washer I used to remove the latex did not faze the original paint. I would rather repaint with a more subdued color. How can I remove the old paint or get latex to adhere? — Neil Wolf, via email
The answer is not as obvious as I thought. Concrete itself presents adhesion problems, because it's porous and alkaline, and walking surfaces are especially problematic. New, unfinished concrete is easy enough to deal with given modern products. Previously finished surfaces are harder.
The red you see may be a stain applied when the concrete was wet, making it integral, or it might be an oil-based coating (in a formulation no longer made), or it may be an early, postwar epoxy. If you can get the floor and steps absolutely clean, with no embedded dirt and no grease, and a surface with no peeling or other paint failure, you should be able to repaint successfully. First, all subsequent latex paints must be completely removed.
Looks like you already did a good job, using the right paint stripper and power washing. If any stains remain, clean with TSP or an oil-stain remover from a company like SEAL-KRETE; rinse, and allow to dry. Be sure all mildew and mold are killed and stains removed. If the surface is very smooth you may need to etch it with HCl acid; you can buy acidic etching liquids from Rust-Oleum and cement-products companies. I think it’s unlikely you will need to do this on a weathered exterior surface. (Please use full safety equipment when using TSP and acid.)
All patching should be done and have cured. You do not need a sealer. The floor must be thoroughly dry. Since you will be priming and then applying two or three thin coats of masonry paint, with 24-hour drying periods, you’ll need up to four days with no precipitation and, ideally, low to average humidity. Apply one coat of a bonding primer to assure compatibility and adherence with the old finish. Whether acrylic or oil-based, this should be a formula specifically for masonry; it may be labeled “elastomeric." Use a roller; do not spray.
I was surprised to find that epoxy coatings, the most durable for concrete walking surfaces, are not recommended for previously painted concrete, especially if the paint was oil-based. So that means you should use a masonry finish paint specified for floor surfaces, one compatible with your primer. Both acrylic latex and oil formulas are available. (Sherwin-Williams Porch & Floor Enamel, for example, is latex and can be used on concrete.) Use a spray gun or brush and roller. Apply at least two thin coats, 24 hours apart. Note that optional paint additives can lend texture for “grip” (safety). Please consult with a knowledgeable person at the paint or masonry store when you buy products.