I've been using phosphoric acid to treat the steel surface without having to do complete rust removal first. I just get the worst rust off, and clean, then at over 50 degrees F., apply the phosphoric acid, soaking the surface completely, allow it to dry completely, with plenty of time and kept above 50 degrees (somewhat higher than 50 degrees is probably better). Then the surface has its surface completely reconditioned and converted into a nonrusting form of steel/iron.
I've also used Corroseal which contains phosphoric acid and also contains a tough sealer to prevent re-rusting. The Corroseal is painted onto the cleaned surface (probably should be cleaned more thoroughly of rust, than required for phosphoric acid alone) then rinsed and dried. Then applying the Corroseal at above 50 degrees F., or a bit higher temp., allow to dry completely, and then, look for places where there may be untreated gaps or places requiring more Corroseal or simply apply a second coat of Corroseal. This seems to be a very good method for protection.
The other person answering here mentioned a product (Bulldog), that I'm not familiar with, but that might be very good, just have to figure out how to use it if it's used in combination with other methods such as mentioned here. I'm sure Corroseal may require complete wash/rinse after sanding or brushing the surface; but it also may be okay to paint the bare metal with phosphoric acid, allow to dry thoroughly, and then apply Corroseal. That's something to check with that manufacturer, I'm not sure.
Also there''s the question of using "Bulldog" for surface paint adhesion on top of the Corroseal treatment. Either of those manufacturers might be able to answer that. I've never looked into it because I don't care about the surface color of my vehicle.
All this hassle can be avoided if we start making our own molded carbon fiber composite replacement panels, but you still have to bolt onto a corroding steel frame & structural steel parts so there's always a need to deal with rust, until we have structural carbon fiber composites used in car/truck manufacturing generally.