I use a lot of that, I haven't seen a lot of what I've used it for long term (that's been exposed to the elements anyway) but on our Durango I replaced a fender about 10 years ago from a deer strike and I slathered POR15 on the inside of the replacement fender to (obviously) protect it as long as possible and recently I was in there and was looking around, it seems to have peeled on the inner reinforcements behind the battery and I see a good bit of (hopefully only) surface rust on the back side of the fender