What type of grinding wheel should I get for grinding off the bottoms of rocks? (I do rock painting & sometimes need to grind rock bottoms so they will sit straight.) I have an old dinosaur bench grinder & the original wheels (6" by 1") worked great but I have worn them down. Recently bought a new wheel (coarse grit) & it doesn't work worth beans, so obviously there must be different types but I don't know what to ask for. Any help would be appreciated.