I pulled up the floor tiles in our bathroom that a previous owner put in. In many cases the tile came completely off, leaving all of the cured thinset on the wonderboard. How do I remove this thinset so I can tile over it? I've tried chipping it with a putty knife and hammer, in some cases that works, but it's taking forever. In other cases it has pulled the top layer of the wonderboard up with it.... What's the best way to remove the thinset from the wonderboard?