Rust is a natural process that occurs when metal is exposed to oxygen and moisture. If you want to paint to prevent rust from occurring, or if you need to seal rust to prevent it from recurring, you will need to apply specific types of coatings in the correct order, or you will almost certainly see rust begin to develop. If you need to remove old rust before you begin the application process, you should also know the proper tools and safety gear required to do an efficient, thorough job.
Don protective gloves and eyewear.
Remove any chunks of rust by scraping them with a wire brush. Smooth smaller, jagged bits of rust using 80-grit sandpaper.
Apply a coat of red oxide or zinc chromate primer to the surface, effectively sealing existing rust and preventing new rust from forming. Use a paintbrush manufactured for applying oil-based paints. Allow the primer to dry for two full hours.
Wash the paintbrush with mineral spirits.
Apply a coat of oil-based enamel paint to the primed metal surface in the same way you applied the primer. Allow the paint to dry for two hours. Apply another coat if you can see the primer bleeding through.
If you are planning to paint a galvanized metal surface, use a galvanized metal etching primer in place of a red oxide or zinc chromate primer.
Do not use an ordinary oil-based primer in place of a red oxide or zinc chromate primer, or you may eventually end up with rust.