PPG makes products for this.
one to etch and remove oxidation and one to make the surface more stable so oxidation won't happen under the paint. the 2nd turns the aluminum a slight yellow color.
I went one step further and lightly sanded the surface first. Its been 2 years and the aqualu tub seems to be holding paint well.
I got a gallon of each and used about 1/2 of each jug to do the cab and truck bed on a 45. I think both gallons were about 60bucks.
most autoparts stores that sell and mix paint carry PPG products.
I probably can find out the numbers if you need them
