I've painted lots of aluminum.
You need to have it bare and clean. Then get from your paint shop some conversion coating chemicals.
Dupont makes the stuff. first you clean the surface with "alumiprep 33" to acid etch the aluminum. You scrub it with maroon scotchbrite while you do this and rinse it off. The water should sheet off not bead.
Once the aluminum is spotless clean you then use a product called "alodine" dupont 226s I think. It causes a thing called conversion coating. Which make the aluminum think it is surface oxidized. So it will not continue to want to corrode(strange I know)...
This alodine is also makes the paint stick. But you want to ensure you do not use too much, you basically wet the surface with it for up to 3 mins and then rinse it off. So you get a golden appearance to the aluminum.
The first paint you use is an epoxy primer. NO etching primer allowed. Top coat with what ever you want after.
The aluminum should not be sanded. It should just be scotchbrite cleaned with the alumiprep.
Do not not use POR 15. It will peel off, and there is no reason to use that product on aluminum.