As others have said, the UV will kill POR-15's color if you don't do something about it. I painted my trailer last summer with it, knowing it wasn't "UV color stable", and just did a little touch up a week ago on the trailer.
The color change is quite dramatic, and I even used semi-flat black. The "new" paint is MUCH "blacker", where the area thats been painted for over a year is more "gray". Seems to be sticking pretty good, but IMO, it's just paint, and it probably won't stick much better than a spray can. Goes on thicker when you brush it, but thats probably the only advantage.