Zinc chromate primer for bare alli. The industrial stuff from Napa. You can get it in a spray bomb and it goes on so you can still see through the finish- it chemically keyes into the aluminuuim. I then Sprayed on a layer of 3M Ultra-Schutz to cover up all the dings and scratches. The top coat was a limestone marine industrial enamel put on with. That stuff is tough as nails and has not scratched off while brushing with small tees and shrubbery.

You can't get any more outdoor DIY than this.