I never tile over tiles. However, in this case I have a small area that is a shop front and I'm told there is a void behind. I know I could remove the old tiles and assess the substrate, reboarding it if removal of old tiles causes damage, but the existing tiles are sound and I'm tempted to go over them. The customer has asked me to. I'm planning to key in the old tiles with a few notches using a grinder, bal prime it, then rapid set fix some ceramics. Any advice would be appreciated.