I would like to add, that although the initial cost of a grinder may seen high.....some rock isn't cheap, and you will easily make your money back on rock that is not wasted, if used right they last a long time. I recomend #60-#100 grit wheels.
I have also found you can use a wet saw blade for grinding, if held at a 45 degree angle to the blade. This is something that one would have to experiment with to get the hang of it, but it works quite well!
Brian