scrape as much of the loose paint off by hand tool cleaning, pressure wash, sand and feather in edges where the loose paint has failed. painting aluminum can be tricky due to passivation or its abillity to form its own protective coating of aluminum oxide. the paint that i have used and that i recommend is sherwin williams dtm wash primer. if the coating has failed in a few areas then you can just spot prime those areas that are bare metal. before coating over any of the bare metal, you will need to solvent clean those areas with lacquer thinner to remove the aluminum oxide that has already formed after pressure washing. after your areas have been spot primed, prime everything with one coat SW dtm primer finish and one coat of dtm finish. i think that the primer finish may be used as a top coat if two coats are applied, possibly saving you from buying excess material. SW also claims that their super paint or duration can go direct to aluminum. i have never used these products for this and have always felt that it was a must to use an etching primer on aluminum for a good mechanical bond.