I have removed the old tiles from my bathroom (from 1973) and there is a very thin black residue on about 90% of the concrete. I'm going to put in ceramic tiles and need to know if I need to somehow remove this residue. It is almost like a stain, or something VERY thin. By the way, the old tiles were under some self-stick ones. While removing the self-stick tiles these other old tiles just lifted without any problem. I sure would appreciate any suggestions or information that could be shared about this.