did mine with some rust treatment and normal cleaning. been about 3 years and I would like to tear it up where it has come up and put on a fresh coat. I treated some areas that showed surface rust with some stuff from the parts store. As usual it has returned. I still like it, but I found there is some maintenance just like paint. I would be curios if POR15 would have made a difference.
I used some stuff called hippo liner that was roll on. It looks fine and has not faded/turned white. I didn't see any reason to have it sprayed. Also I found that it started to cure too fast in hot weather and went on sloppy. I would try to use it at 70 degrees if possible.
other possible problems are noise, but since I never have my top on it doesn't really matter to me. This was on an 1985 4runner and not used for the exterior body panels, but it turned out real nice on the dash