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 (trisodium phosphate) 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. To check, tape a piece of relatively thick, clear plastic, such as a three-inch 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: The surface is too smooth and dense. Installers typically trowel concrete on porches and floors to bring up a layer of very fine sand coated with cement paste. This leaves the surface far denser than the concrete farther down in the slab. When concrete is out in the weather, the surface wears off over time, which is why you can often see exposed sand or even gravel in old concrete walkways and patios. On a porch, though, the surface might be nearly as dense and uniform in color 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, which would also keep paint from sticking, 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-to-rough 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.

However, the etching step needs to come after you clean the concrete. TSP and TSP substitute work equally well for this purpose, according to a technical-help person at the Savogran Co. (800-225-9872; savogran.com), which makes both kinds. A one-pound box of powdered TSP is just $3.96 at Home Depot and would probably be plenty, given that a half-cup mixed into two gallons of water would clean about 800 square feet. If you’re using a pressure washer, a quart of Liquid TSP Substitute Cleaner, for $5.48, would be easier to use and would clean about 1,000 square feet.

For etching, you’ll find a confusing array of products, including standard muriatic acid as well as products such as Klean-Strip Green Muriatic Acid ($7.84 per gallon at Home Depot) and Klean-Strip Phosphoric Prep & Etch ($15.78 a gallon). The “green” muriatic acid has a lower concentration and isn’t strong enough to etch concrete that was troweled smooth, according to the company’s technical-help person. It is a good option if you want to etch concrete that feels a bit rough, however. Phosphoric acid works on smooth or rough concrete, but you don’t need its big benefit, which is its suitability for both concrete and rusted metal.

With any etching product, it’s very important to follow all safety precautions. Wear a full- or half-face respirator with acid-resistant cartridges, goggles, chemical-resistant gloves that cover your forearms, and rubber boots. Have a plastic watering can to apply the product and a nonmetal broom or handled scrub brush for working it into the surface. A pressure washer works best for rinsing, although you can also use a hose. Read the complete label before you even open the container.

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, on the other hand, you found that 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 labeled as suitable for going right over previously sealed concrete, provided you clean the floor, scuff-sand any shiny areas and scrape off any sealer that is peeling. (“Wet look” concrete sealers form a surface film, which can peel, while penetrating sealers, which don’t alter the look, never peel.)

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. On the Behr website, only 62 percent of the 52 reviewers said they would recommend this product to a friend. The average rating is about the same on the Home Depot website; nearly half of the more than 840 reviewers gave it five stars, the top rating, while about one-fourth gave it only one star, the lowest. So your odds of being completely satisfied vs. totally frustrated could be 2 to 1. Many of the complaints, though, deal with using this product on garage floors where car tires stress the finish, so on a porch you could have better odds of being happy.

Still, problems with painting concrete are numerous enough that no matter which finish you choose or how careful you are with the prep steps, it’s still smart to paint a small area, wait a while and make sure the finish sticks. Unpainted concrete is always nicer looking than concrete with paint that’s peeling.