I'm an overnight maintenance supervisor at my target and my boss wants me to clean up the grout back to it's light gray color. It's practically black now. I'm using a pressure washer but it isn't cleaning it up. The tiles look good but after a day or two the black grout starts to spill over onto the tile and it just looks really bad.

I've tried the different nozzles, and even the turbo nozzle doesn't do it. I've tried cleaning it with floor stripper and that doesn't do it.

The best outcome I've had was getting on my hands and knees and cleaning part of it with a rough green scrub pad and the regular soap we use here and that brought some of the tiles back to their original color but it took over half an hour just to do a very small section.

Any ideas on what a better solution might be?