Make sure they use a VERY soft medium and to minimize warpage from heat during the blasting process. Crushed walnuts works well. BTW you'll then have valve covers that stick out like sore thumbs as compared to the (not blasted) engine block. As for the mating surface don't do anything to it unless it's warped in the first place. Don't mess up something that's not broken. If it is, I've done something as simple as placed it, mating surface down, on a large piece of affixed sand paper, that's on a level table, and gently sanded it back and forth until it takes a little of the warp out. Now, granted I've never done this to a DMC valve cover, but it's worked on other warped mating surfaces.