It depends on type of boat and construction. But if you want paint to
adhere to AL then you MUST either Sandblast, Acid Wash (like Aluma-Prep
33 and
Alodine1201) or otherwise sand to provide a oxidation free surface for
paint. I also paint with either zinc chromate or zinc primer to avoid
galvanic corrosion. I do work on AL boat and I know a mistake means AL
turns to swiss cheese. Also your choice of bottom paint are different
since you must absolutely NOT use anything with Copper in it. There are
barrier methods with epoxy coats but none really work more than a year.
If your boat is more than 12 feet long - you need to do some serious
research into subject.