Rust converter before primer OK ????
 

Is it OK to use any of the rust dissolvers, rust converters, rust encapsulators, POR15 before applying primer to a body which has superficial rust ?

Or what steps do you need to take to get rid of this type of rust and avoid it to reappear ?

I have used POR15 on bare metal then scuffed the POR15 to prime.

You have to kill the rust (products such as naval jelly, rust mort, ospho, metal ready, molasses, etc) before you apply any coating.

All the coating does (POR 15, your preferred primer/paint system) is encapulate the killed rust to prevent moisture and air from reaching it.