Bob is spot-on. I shoot epoxy on aluminum more than any other metal and I typically just toss it in the blaster first. I use recycled glass in the blaster (the multi colored broken bottle bits and pieces) because it's supposedly cleaner, tremendously cheaper than AL Oxide, and it's rated at 70 grit if I remember. However, I only blast enough to dull the aluminum and I keep it moving very quickly because a lot of what I shoot is only 20 thousandths thick.
If you're going to sand your pieces instead, they recommend 80 grit. Personally, I will typically go 80-120 on Aluminum depending on the grade. THE ONLY exception to using heavier grits that I have run into is on DEAD-0, or annealed aluminums which can be extremely soft and almost "smear" when you try and rough them up. Just give those materials enough scuff to equal what the course grits would on hardened aluminums. The epoxy primer bites great and you won't have any issues with it letting go.