I'm a boat captain, and on a steel boat in salt water rust is always a problem.
The #1 (by far) method to deal with rust is to remove it by sandblasting. Take the metal down to gray. If you do that, then treat and coat the metal, the area is stabilized and the rust will not come back unless the coating is damaged.
#2 method is to remove as much as possible by mechanical means (wire wheel, needle gun, scraping, etc) then treat with Phosphoric acid (Rust-Away or Muratic). Done correctly the acid can take the metal down to gray. Then rinse with water, treat with Corroseal or similar rust converter and paint.
#3 would be to omit the acid (because acid is dangerous and not everybody can be trusted with it). Mechanical removal down to "tight rust", then treat w/ rust killer and paint. The rust killer may or may not kill it all...and it may come back at some point. Diligence is key here
I would recommend this stuff for a coating
A quick web search will dig up a supplier in your area.