We used to clean rust out of motorcycle gas tanks with radiator flush and a package of BB's. Seal the openings, pour in the flush and BB's, close the lid and shake until your arms drop off. No reason you can't do the same with the metal can.
POR15 is not the only product for sealing tanks, but I think they all work the same. I would still eliminate as much rust as possible before using any sealant or it will continue to rust. But I would not try to seal the inside of the can unless it is approaching terminal.
But before I did any of that, I would stop in at the hardware store for a new can.