Q: We have a 3000-psi concrete slab six months old that is already oil-stained. The oil doesn't come off with muriatic acid. How can we get it off?

A: Acid isn't normally effective in removing oil stains. Scrub with a strong detergent or 5 percent sodium hydroxide solution, or trisodium phosphate and rinse thoroughly. Wear rubber gloves and particularly avoid getting sodium hydroxide or trisodium phosphate on the skin. If skin is accidentally splashed with it, wipe dry, swab with dilute vinegar, rinse and dry again.