I honestly do not think you can get any better than good old fashioned Rust-o-lium oil based paint (spray can or liquid). Any water based paint won't hold up on metal. But Rust-o-lium has always stood the test of time for me. It has a rust inhibitor and you can spray directly on the rust and it will kill it too.
I'm in the process of spray painting all the under carriage stuff on my travel trailer this year. I just sprayed my entry steps tonight and the exposed part of the main frame you can see behind the steps. I did the stablizer jacks and bumper and tongue last week. Black spray Rust-o-lium will protect for years and years. Nothing better in my opinion.