I'd like to treat my old previously painted outdoor concrete steps with a green stain to accent the house color. Some of two former colors are still stubbornly visible after applying citrus paint stripper, scraping, and scrubbing with tsp. I'm concerned that the remaining colors will ruin the stained effect we're after. The old paint is faded and smooth enough to paint over, but not invisible enough for stain. Can someone please advise a diy wannabe on how to remove the last vestiges of past paint? Thanks. Zoe
2Maybe rent a pressure washer. 3000+ pounds usually does a good job, will clean the cement well also for painting.– crip659Oct 23, 2022 at 22:50
Try gasoline, no smoking.– RuskesOct 24, 2022 at 0:14
heat gun can work too.– Fresh CodemongerOct 24, 2022 at 1:11
With a few additional thoughts, I see 3 answers here in the comments that should be down there in the answers.– FreeManOct 24, 2022 at 11:37
Try a stiff wire brush and power washer. However be careful, if it's soft old concrete, the wire brush can give you valleys.
If it's old, soft concrete, the pressure washer could leave even bigger ruts and valleys. It's amazing what 1500-3000 PSI can do. That said, I cleaned my inlaw's concrete barn apron (only ~30 years old at the time) with a pressure washer - set it to a wide fan pattern, used a low angle, and didn't shoot it right at my feet - I let the water run for a distance before it hit the ground, and had no problem at all...– FreeManOct 24, 2022 at 14:28
I had a painted concrete wall with many layers of ugly paint colors that just laughed at a pressure washer. I made up a strong caustic solution of lye flakes in water, thickened with corn starch. The solution gets very hot as you mix it, and this helps to dissolve the corn starch. The purpose of thickening the solution is to keep it in contact with the paint instead of just running off. Apply with a synthetic bristle brush; natural bristles dissolve in lye. Leave the goop on for about 30 minutes.
The caustic solution softens the paint and causes it to bubble and release its hold on the concrete. After rinsing with a garden hose to remove the caustic, a pressure washer effectively removes the rest of the paint and leaves a clean concrete surface. You may get the same effect with a stiff brush instead of a pressure washer.
A few areas with very thick paint had to be treated twice on my project. Grass growing nearby didn't seem to mind the high pH.
NOTE: Citrus paint removers were developed as a safer alternative to caustic lye solutions, which have injured or blinded people in times past. If you go this way, be aware that you are working with dangerous chemicals. Read the precautions on the package. Protect your eyes and skin. Keep children and pets away. Keep lye solutions away from aluminum. Rinse well.