I have used a few things, but a new or clean stainless wire brush is the ticket. I also purchased some aluminum cleaner from the local welding shop expensive but it works great. It eats off the oxidation. But Easy off in the yellow can works just as good, and it is much cheaper. Make sure you buy the one that doesn't need you to preheat the oven. Oh and the stainless wire brush is a must. Aluminum will start oxidizing within a couple of hours. I always clean it off really well, then clean it again just before I do the welding.

-Dave

-Dave