There are rubbery sanding pieces that will slowly grind something down. I can't remember their part # and I'd have to feel one to know rather than see a photo. When I was restoring serial numbers at the crime lab, that's what we used to make the area a smooth surface before applying the acids. Anything else took it down too fast. We didn't get aluminum too often, but we did see it occasionally - same procedure.
Most "machine shops" that cater to working with auto-repair shops for cylinder boring, etc... are generally more than willing to help someone out, at least from MY experience. Before I had money in my pocket, I scraped together my pennies to buy a 6-pack of Bud and made a swap for my "small projects" like boring/enlarging a lensboard(s).
Usually they were more than willing, I'm sure its an "industry-wide practice/standard"