Trying to remove this spot on ceramic tile. I believe it's from one of those commercial door stops with the rubber feet. It lines up exactly with the door stop, but not 100% sure. I have tried all sorts of chemicals (groutmaster, acid, degreaser, goof off, graffiti remover) and equipment (175, spinner, turf pad, brush, melamine, scraper) but have not been able to put a dent in it. Any ideas on how to get this off? Thanks in advance.