I don't like POR much. It is great on something like a suspension arm where you can't be bothered to clean it up really well. Ir is ultimately what it says on the tin; POR Paint Over Rust. It doesn't adhere at all well over painted surfaces or clean steel. The other issue is that it is very sensitive to UV light, it degrades badly. It is also very intolerant of being overcoated with other finishes.
For rust corrosion inside box sections Bilt hamber wax is about as good as it gets. http://www.bilthamber.com/corrosionprevention.html
If I want a shell completely stripped then acid dipping works well. It completely removes all corrosion, underseal, paint and seam sealer. This Volvo PV 544 Sport is currently in my workshop. I got it dipped to remove all the paint and rust. I then welded in new floors, transmission tunnel and repaired the crahed rear end. It then got re dipped to remove flash corrosion and electrophoretically coated. E coating is complete submersion in a vat of primer. SPL in Birmingham do it. After that I'll epoxy prime it, prep the body work, primer fill it and finally top coat it. After that I'll inject all the box sections with Bilt hamber rust preventative wax. Cheap however it isn't.
