Painting over Rust is often seen as "impossible", or "it will just come back" etc etc. There are several things to be aware of when painting "over" rust.
1. Ensure the metal is thoroughly degreased and decontaminated - Steam cleaning is ideal
2. Descale. You may need to actually chip away the rust using a needle gun, chipping hammer to reduce its thickness.
3. Apply ENOUGH product. so if your going over rust with aerosols, give it 4 coats instead of the usual 2.
4. Our direct to Rust paints either use Stabilisation technology, or Vapour release technology- So they actually work!