LD;
i would think that your grinder would work as long as you can work down the entire surface evenly.....but go slow.....most slate is pretty solf material and will grind away quickly.
you will probably still have to sand the surface smooth after the grinding.
What kind of striker are you going to use with it?.........I made one out of a Locust which is the hardest wood that i know of.......worked well with the slate.
Is this slate, your using, is it like a floor tile?
TCB