The best way to prep it, is to etch the surface by abrasive blasting. Use a lower air pressure, keep the nozzle about 6 inches away from the surface and keep the nozzle moving. If it's to close and not moving around, it can "eat" a scallop in the surface. After it's done, give it a few minutes soak in Alodine. Then wash it with soapy water. A hair dryer will work for drying it and it's ready for painting.
The Alodine will give it a nasty yellow look but it's a good prep on Aluminum for under paint. Depending on who you talk to: It's almost as hard as regular Anodizing, it's just as hard or it's a little bit harder. Keep yer powder dry, Mac.
Tuff-Gun Finishes. The Name Says It All.
Mac's Shootin' Irons
http://www.shootiniron.com