IMO paint over rust "Rust Killer" products are pure snake oil for the lazy man who wants to avoid doing things right. It's quite simple, you can't stop rust, you need to COMPLETELY remove and eliminate it (cut, sand, grind, blast, which ever).
Once it's gone treat it with a metal conditioner such as the above suggested Ospho which is a watery blue colored liquid that looks like Windex. It contains a mild acid which eats micro rust and leaves a phosphorus residue giving BRIEF rust protection until a good protective sealer/primer can be applied (DP
LINK products, etc.). Then paint as normal. When PROPERLY finished you should never have rust issues again as long as you are alive.
Now, if repairing an off road rig, daily driver low budget car, lawnmower, etc. and you don't want to spend any real time or money getting it to look decent (for a while) I'd use Zero Rust, POR15, Spray on Bedliner, or similar, just don't expect any miricles if you paint over rust.
