Q: My indoor patio was stained green with oil-based paint 68 years ago. I put on two coats of Valspar floor, deck and patio for concrete, but it's peeling. What do I do?

A: Your question raises a few more. Did the old paint peel, too, and get touched up over the years? Or was it fine for all those decades until you repainted? What finish did you add? (Valspar makes both oil- and water-based versions of its porch and floor paint.)

You don’t say whether your indoor patio floor is concrete, but let’s assume it is. If the old paint peeled all along, that’s most likely a sign that moisture is coming up through the slab. Tape a piece of relatively thick, clear plastic, to an area where the paint has peeled off; even a three-inch square would be sufficient. If moisture droplets appear over the next day, no paint will last very long without peeling at least in spots. Unless you want to go through the hassle and expense of grinding off all the paint or stripping it with chemicals, you might just accept that you will need to scrape the loose paint periodically and touch up those areas.

But because you say the original paint was there for so many years, it’s probably more likely that the peeling began when you repainted. In this case, you can learn a lot about possible causes by carefully examining some of the peeled-paint chips. If they are smooth and clean on the back, the old paint might have been coated with wax or a sealer , or the surface was too slick for the new paint to grip well. If the chips are dirty on the back, the surface wasn’t clean when you repainted. In either case, you might need to take the blame for not reading and following the instructions, which say to thoroughly scuff up the surface and wash with TSP (trisodium phosphate) or a TSP substitute, which etches the surface and removes wax and grease.

But if you see some of the vintage green paint on the backs of the paint chips, the cause could be even more maddening. It’s possible that you used Valspar Oil Porch and Floor Enamel, said Mindy Lemieux, technical service representative for Valspar and Sherwin-Williams, the paint company that bought the Valspar brand in 2017. “Our oil-based products are very intense,” she said, characterizing the paint as “hot” in terms of its solvent content. She said that when it’s used over old oil paint, “it can act as a stripper.” She said the paint is “meant for bare concrete only.”

But neither the product page on the Valspar website nor the instruction sheet that can be downloaded from that page include this caution, although there is a warning against using the paint on exterior wood decks, garage floors or other automobile parking areas. The “recommended uses” include “properly prepared concrete,” and the surface preparation section says, “The surface must be clean, dry and free of all loose or peeling paint,” which implies it can be used over existing paint. The additional details for use on concrete mention procedures for new, unpainted or smooth-worn concrete but do not cover previously painted concrete. Who would guess that the absence of details means not to use the product?

Whatever the cause, Lemieux suggested scraping the peeling areas, then doing what the company would have recommended before you repainted. “We would have suggested scuffing very aggressively, even with a wire brush, then wiping it down and using a water-based paint,” she said. After wiping off the debris but before repainting, you should wash the floor with TSP or a TSP substitute, rinse thoroughly, and let the surface dry.

When you’re ready to add paint, brush it on over only a small area. Wait to see whether it stays intact. If it does, you can proceed to paint the whole floor.

But if it still peels and the moisture test turned out fine, seek help from an experienced painting contractor. Or call Valspar’s customer service number (800-845-9061) and tell them you need help. Grinding down to bare concrete and starting over might be your only option.

Or, invest in indoor-outdoor area rugs.

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