When I replaced my hog troughs, even though they were galvanized, I used KBS Rust Blast and then Rust Seal inside and out. I then used the KBS Blacktop on the outside surfaces. Since I also was doing a frame off, I also used the Rust Blast, Rust Seal and Blactop on the frame.
The Rust Blast is a Phosphoric acid the neutralizes any rust the sand blasting missed.
As a suggestion, I used the Satin finish for the Rust Seal and the Gloss for the Top Coat so I could see if I had missed areas when applying the top coat.
KBS also has a product called Cavity Coater. It comes with a 36" flexible tube and 360 degree tip in an Aerosol can. You can use inside the frame.
I actually think the KBS products are better than the POR 15 products, but that is my opinion.