When I used POR15 on mine, I scotch brite the bottom of tub because it was new and support said to scuff it as there was no way I new what type primer was used. Tub turned out great and going on about 14 months now, no peeling etc. On my frame that was a different story, Wire wheeled, needle scaler to get the undercoating completely off and then went over it with scotch brite.
I still had some rust spots on the frame and was not worried about them. I did use cleaner degreaser and the driveway acid wash product available at Lowes, let it completely dry and then went to town with my brush. Use a high quality brush. I bought a gallon of POR15 and after doing entire bottom of tub and all of frame/differentials, I still had 1/2 gallon left.
Luckily my brother in law needed some to finish is frame off resto so none went to waste. And the comment above about getting it in the sealing portion of lid and can follow that because if you bang the lid back on and have POR in there, you will be using a hammer and chisel to open it again.
POR15 doesn't care about there being rust, I just made sure I did not have any loose pieces is why I wirewheeled and needle scaled. PO was in love with undercoating because he owned his own shop. Good luck....