Try POR-20 (from POR-15) aluminum primer.
I used their Hardnose paint. It seems to hold up great to strap chafing and UV, but does abrade just a bit all summer on shuttle trailer rides--between the frame and raft tubes.
Aluminum is a bugger to paint. make sure you scuff sand, sandblast, or chemical etch it to give you more surface area for the paint to bond to.