I decided to go with POR 15 for all of the inner surfaces and a etching primer, light coat of paintable rubberized undercoating and then a coat of the same color as the car will be for the underneath the body, inside fender area, and frame rails. I had concerns as to anything else sticking to the POR and the reported fading due to UV. I decided on this while I was scraping all of the old undercoating off the car. Everywhere on the car that still the primer/undercoating/paint intact the steel was just like new. So if it worked after 40+ years I think it will last until the next owner does a restore in 40 or so years.

I've used POR for years and REALLY like the stuff. Rust Bullet sent me some product but I haven't had enough time to fully develop an opinion on it yet. If you overcoat POR within 6 hours or so you should have no adhesion issues. If you wait longer - especially days or more - then you should either scuff it with 220G or use their Tie-Coat primer. I use it on the UNDERSIDES of all sand blasted frames and parts. It looks great (OEM), self-levels when brushed and is tough as fricken nails. Just make sure anynew/smooth metal or painted surfaces have been well-sanded!