1. Use a wire wheel on a grinder to get all the loose rust / oxidation off.
2. Treat the remaining metal with something like
Ospho which kills the rust. But you're not done yet...
3. Paint the metal with quality paint. Not just primer, because primer is not going to seal it well enough to prevent rust for long. Use a strong rust-preventative paint. I like
Hammerite which is available in spray cans as well as pints/gallons/etc. Others like POR15.