I did attack the rust in the rear bath area a bit with a wire cup brush to get rid of any scaling. The POR seemed to have really good adhesion, even in spots with mostly intact original paint, so it didn't seem worth it to switch to a different product. I may just use Rustoleum up toward the front of the frame, where there doesn't seem to be any rust at all.
Wear really good gloves when you use POR 15... it tends to dye your skin. I made the mistake of using a styrofoam bowl to hold a bit of it while I was working, and it ate through it. This added interest to the cleanup.
I've attached a "before" picture of the rear frame...