Jim's right... you want aluminum oxide, if anything. Silicon carbide is bad. Also, FWIW, I'd be cautious of too much sanding. I believe those 0.016" skins are 2024-T3 alclad - a very thin layer of pure aluminum on the surface of the 2024 alloy. I would hazard a guess that you have already removed much of that protective layer. A good protective primer is going to be very important. I'm sure you probably know that already, but figure it doesn't hurt to mention it.