Bill,
Proper pretreatment for aluminum painting takes place in at least a 5 stage pretreatment system. The system would consist of the following stages:
1: Alkaline cleaner (aluminum-safe, or mild etch)
2: Fresh Water rinse
3: Iron Phosphate solution containing fluoride
4: Fresh Water Rinse (Recirc DI Optional)
5: Fresh Water Rinse (Virgin DI optional)
Choice of cleaner depends on alloy and soils to be removed. Do some research or ask in the shop. The fluoride is the key. It etches the aluminum surface to provide for good paint adhesion. If you reduce the stages, then you reduce the chances for success.
Cheers,
gr2vessels