How thick is the metal? If the metal is sufficiently thick, after removing the rust as much as possible I would use phosphoric acid (ie Ospho). After the acid has done its thing, wash off the acid. I'd use ethanol or isopropyl alcohol with the highest alcohol percentage I can find. Then I'd pain the treated area with boiled linseed oil. When that's dry after a few weeks, I'd paint over it with an oil-based enamel or antifouling marine paint. The rust will eventually return but you can slow its progression down considerably.