Sometimes you gotta do some jobs out of the box. We did a tile job today (neglected grout), it was black! We mixed some muriatic acid with water and sprayed it on the grout lines, let sit scrubbed, then turboed. Some of it we had to go over! Grout was Shot, we even color sealed some areas to blend in. It is a rental unit, but the customer was happy, not sure our lungs were!