Like the others’ have said, the aluminum is porous and will absorb oil, especially lower units that have been exposed to everything! It is very important to get it as clean as possible. I use acetone and a stong degreaser like Castorl Super Clean. Then I use Rust-O-Leum self etching primer, followed by a few coats of their sandable primer. The self etching is only there to give tooth to the next layer of primer. Let dry, sand and repeat if needed until you’re satisfied with the surface and topcoat.

Steve