Tim Adams recipe for inner panels, floors, frames etc, sandblast if possible, or sand and wire brush, acid treat, and primer with Rustoleum rusty metal primer, paint with Rustoleum oil based paint, or use rubberized undercoating end of story.
POR 15 is another high priced encapsulator, paint over solid rust, not flaky loose scaly rust.
The truth be known, if you clean up rusty metal, and paint all sides, and let cure, all is well.
No air ( o2), no moisture, no rust process, or at least it is delayed, until some void will allow air, water, salt to penetrate to start the corrosion process.
I have painted some itmes, that still had solid rust, and it is still holding up very well, as my car is driven in summer only, and not in sloppy,slushy, salty, winter weather.
I would use Tim Adams recipe, or a paint called Zero Rust is pretty good, and 1/2 the price of POR 15. POR 15, can delaminate too, Rustoleum/Tremclad, sticks like $hit to a blanket, once cure, the trick is to get have good prep, and good coverage on all sides, so as not to allow penetration of salt, crud, rust and moisture to attack.
How does that sound Don........