This is how i usually fix rust.
1. Sand back the effected area making shore all rust scale is removed.
2. Then use a rust converter to remove any left over rust or oxygen left in the steel.
3. Etch primer the area, 2 coat. The hi levels of zinc in the primer will help stop it oxidizing again.
4. Then a "Hi-Fill" primer.
5. Sand and paint.

I would highly recommend POR 15 as draws moisture out of the steel as it cures.
It also come with an instruction for surface prep.