I want to acid stain an existing, older concrete slab. What’s the best way to clean the concrete so the stain will take? -David
Cleaning an old concrete slab so it will take an acid stain can be very difficult, but it is possible. The main thing is to NOT use any kind of acid wash or etching solution (including muriatic acid) to clean the slab. The stain will not take at all if you use acid as a cleaner.
A solution of TSP (trisodium phosphate) and a lot of elbow grease with a scrub brush would be your best bet. Using a scrub brush with a long handle makes cleaning much easier. Another option would be to use a pressure washer to clean the concrete slab. For grease stains on concrete, try scrubbing the stain with lacquer thinner or mineral spirits.
Even with your best efforts, you may wind up with patches on a concrete slab that simply won’t take an acid stain due to low lime content in the concrete (lime is what causes a chemical reaction with the acid stain). In those cases, you may have to resort to applying a faux finish to the spots to try and match the acid stain in the surrounding areas.
Good luck with your project,