If it's just the wrought iron gate, or Aunt Mabels letterbox then there used to be a product called "Ferropro Rust Converter", and that was pretty good stuff. You scraped back the excess rust and then hit it with a wire brush so that only a thin film of surface rust remained, then you painted it with Ferropro and left it to dry. It turned the metal (and rust) black, and then you painted over it. I had a lot of success with it over the years on things around the yard.
After using Ferropro Rust Converter on the exposed metal, I used Plastex (available from motorcycle shops) to fill and repair the cracks. It's basically powdered ABS with a solvent that bonds it to the original material, I also use it on bike fairings with great success.