Just thought I would throw in my .02 on the topic of painting aluminum.
Does not matter how well you sand alum., you need to etch it, or etch primer it to keep the paint sticking!
Alum. is an oily metal that leach out when sanded. (If you do not use etching primer, clean well with methylhydrate after sanding to remove residue)
Best to use an etching primer made for alum. Plus, if you go from primer to top coat, the best bond is a chemical bond.
Don't wait for it to dry and sand then top coat, better to just give it enough time between coats to flash off and then top coat! Gives you the best bond.
Primer and top coat must be compatable to do it this way, but strongest bond if you do.
Just make sure the alum. is all smooth before you start and the finish will be smooth too.
Every application is different so variances in product and procedure will apply.
Up here in the north coast area, for metal trailers you have to either Galvanize your trailer or use cromate primers to make them last (wich in your location would mean last forever

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Rob