I think if you scuff up the surface, the POR 15 will be fine.
I've used this stuff on the wife's rear wheel well lip where there was some rust starting. It put an immediate stop to it. Just be sure to coat both sides of the steel.
I've used it on my car's rear wheel well in the same place to prevent any rust in the future (everyone should do this IMO).
I've also used in under the car on the front sway bar, frame and anywhere else that can be chipped up by rocks or road debris. It has held up very well so far and I don't expect it will ever come off.
That POR 15 is unbelievably tough. I spilled a few drops on my driveway a couple years ago and the spots are still there looking good as new, despite all the winters, snow plowing, sun, weather, etc.