For polishing after bead blasting, it all depends upon the finish you seek. A highly polished, chrome-like finish is enormous work. Start with 400, then 800, then 1500, then 2000, then run through 3 rouges progressing each step.
If you seek a polished surface to bring some luster back after blasting, 800, 1500 and hit the buffer! Blasting actually can drive some impurities into the alloy cases, so you may need to back-track on some parts if you discover black pitting, or yellowing as you step up the polishes. You might try a quick heating in the oven to help purge the grime, then Acetone, then begin sanding. (Again, if you desire high luster, high polish).
It's a chore and absolute labor of love... I like powder coat because I'm lazy