On an elevated deck job we're doing, ironworkers used black lumber crayons on the deck forms to mark the location of rebar. When we stripped the forms we found that the black marks had transferred to the concrete surface on the underside of the deck. They are very noticeable and unattractive. How can they be removed?
We suggested using a commercial solvent but that didn't do the job. The contractor then found his own solution. He used a 3000-psi water wand and put a mild cleaner in the water. That removed the marks.