I had a couple sets off of old hemis. They weren't severely worn but needed a "clean-up". I mounted a medium grit flap wheel in my drill press and did all the valve contact surfaces. A friend who owns a machine shop said they used to grind them years ago and said I had nothing to worry about doing them my way. Give it a shot. The material is pretty hard so the flap wheels conforms to the shape - unlike a grind stone.