Its tricky to get paint to stick to aluminum, and it depends on the condition of the paint to start with. If the OE finish is in good shape then the prep is not the extensive, but if its peeling with a lot of aluminum corrosion its a lot more work.
go to
www.boatered.comlook for a sticky in the engines section on painting aluminum outdrives, by Geebee. He has it all there.
here in the salt pond, its a losing battle. I just sand, wipe it down with paint thinner, hit the bare spots with Primocon and then top coat it with 2 coats of Trilux 33 anti fouling paint.