You generally want a two-part epoxy
barrier coat, but you may also benefit from some sort of galvanzing layer/zinc primer. You may wish to ask your surveyor
about this, but essentially, the treatment is quite similar to restoration
of car body panels
: remove rust to bare metal, top immediately with zinc-rich primer, coat in a waterproof barrier coat, and then antifouling that won't react with metal.
Devoe makes something called "Rust Bar" and Pettit makes something called "Metal Prep". I've had luck with both, but you should get someone local to you to clap eyeballs on your particular boat and make some recommedations.
I have found this boat very helpful:
It's important to understand why
boats rust in order to properly stop it. Possess this understanding and do the work, and your metal boat will likely last as long as any plastic boat. It's more than just about paint