I'm not surprised the POR15 didn't seal it. You cannot just paint over rust, it will return eventually. You have to chemically change rust with a rust converter and then you can seal it with a product such as POR15, Rustoleum and the like. You have to change the rust on a chemical level. I would treat that whole roof with a converter and then choose the sealer of your choice. Either that, or put a roof skin on it.
