I have used the specific POR product made for the exhaust manifold and fuel tank sealer on my vintage power wagon restoration. So far so good.
However, when blasting off some Por the previous owner had put on the frame, I noticed that it came off in sheets-delaminated- with very little apparent grip ot he metal in places. IN other words if you angles the abrasive blaster it lifted off very easily. I also found some places where there had been a rock chip and the moisture got under the POR and rust spread under the POR so you could not see it right away.
I then did this experiment (which I showed Pictures of on the Power wagon forum) I took one small part and blasted it and painted it with epoxy primer and hardened urethane. The other small part I painted with POR after doing everything they say to the metal as a prep. THen I dipped both pieces in a warm lye solution. In less that 2 minutes the piece with POR was stripped clean. THe other part looked like nothing had happened.
Conclusion: I have serious concerns about using POR on sheet metal, not because it is not very hard (it is), but because of its questionable adhesiveness to the metal and thus protection.
Over on the autobody forum they recommend blasting and painting with epoxy primer and urethane or polyurethane for durability. If that cannot be done, they recommend Zero Rust. Cheaper than POR and supposedly better.