You don’t want to touch the top of the blocks. But you can stone the bottom of the sprue plate. I do this w/ every mold I buy. Except for the ones made by Miha. Those plates are perfectly flat w/ no burrs. I really like NOE molds but the sprue plates always have burrs which will scratch the top of the blocks if not taken care of. You really see the inconsistency in how flat they are when you start removing the bluing from them. But they work great after being stoned.