A CONCRETE SOLUTION
How-to guru Jeanne Huber describes steps to take before applying a final coat.
Q I have an older concrete porch that has never been painted. I will be painting it with patio latex paint. I plan on cleaning with TSP and then applying a concrete-bonding primer. Do I need to acid-etch before painting with primer?
A You’re smart to be careful about doing the necessary prep steps. It’s a lot harder to get paint to stick to concrete than to wood, and the last thing you want is paint that peels, especially on a porch that has survived just fine without paint all these years.
When paint doesn’t stick well to concrete, sometimes it’s because moisture is coming up through the concrete from below. Tape a piece of relatively thick, clear plastic, such as a three-inch (7.6-cm) square cut from a reclosable plastic bag, to an area with no paint. If moisture droplets appear the next day, you might want to keep the porch as is.
The other big reason paint sometimes doesn’t stick to concrete is because the surface is too smooth and dense.
On a porch, the surface might be nearly as dense and uniform in colour as it was when the concrete was poured. Etching is a way to roughen up the surface, allowing paint to stick better.
But etching products work only if the concrete is clean and not already coated. You could easily spot paint if the concrete were coated with that, but sealer might be invisible.
One way to test for sealer is to pour on some water. If it sinks in, the concrete is bare. If it puddles and stays on the surface, assume the surface was sealed.
If the water sinks in, run your hand across the surface. If the texture is similar to medium-torough sandpaper (150 grit is a good guide), you probably don’t need to etch, although it certainly wouldn’t hurt. If the surface is smooth, definitely etch.
But etching should come after you clean the concrete. TSP (trisodium phosphate) and TSP Substitute work equally well for this purpose, says Savogran Co. (savogran.com), which makes both kinds.
If you’re using a pressure washer, a quart of Liquid TSP Substitute Cleaner would be easier to use and would clean about 1,000 square feet (93 square metres).
For etching, you’ll find a confusing array of products, including standard muriatic acid, as well as products such as KleanStrip Green Muriatic Acid and Klean-Strip Phosphoric Prep & Etch. The “green” muriatic acid has a lower concentration and isn’t strong enough to etch concrete that was trowelled smooth, Klean-Strip says. It is a good option if you want to etch concrete that feels a bit rough, however.
With any etching product, it’s very important to follow all safety precautions.
After you’ve etched the concrete and let it dry, rub your hand or a dark cloth over it to make sure you don’t pick up any dust. If you do, rinse again. Then you’ll be ready for priming and painting.
If your porch was sealed, you have a few options: Strip the sealer with chemicals, grind off the surface to expose bare concrete or reconsider your options. Chemical stripping and grinding are real hassles and no fun, but switching to a paint that will stick even to sealed concrete is easy.
Behr Porch & Patio Floor Paint, which seems to be the type of product you have in mind, will not stick to sealed concrete, even with a primer. However, Behr’s 1-Part Epoxy Concrete & Garage Floor Paint is labelled as suitable for going right over previouslysealed concrete, provided you clean the floor, scuff-sand any shiny areas, and scrape off any sealer that is peeling.
But before you commit to painting your whole porch with this or any similar product, paint a small area, and make sure you are happy with the results.
Unpainted concrete is always nicer looking than concrete with paint that’s peeling.