I've used Rustoleum etching primer, works well, use several light coats. If you're priming bare aluminum, sand the aluminum to a bright finish then prime withing 6 hours. If you wait too long the aluminum will form an oxide layer that will protect the aluminum, but interfere with the primer sticking properly.
If I was painting something big (a boat), I'd visit the local automotive paint store and pickup the supplies: etching primer, primer, paint, reducers, etc. It may cost a bit more, but will be easier to apply and produce a better finish. I had to paint the transom of my 18 Crestliner, took the paint codes down the the automotive paint store, and it was a perfect match.
If using Rustoleum paint, use enamel hardener to make the paint cure faster and harder. If you don't use hardener it will take weeks for the paint to cure. Tractor supply is supposed to carry it, Amazon has it in Valspar brand. If using hardener you MUST use a respirator, which you should anyway while spray painting... (Same goes for the automotive paint.)
