I have a bumper I sandblasted, painted with a corrosion resistant two part primer, and topcoated with catalyzed enamel six years ago. They have been stored inside until two months ago. There are now a few spots with a little rust coming through. I'm not going to sweat it or resandblast it/repaint until it gets to looking bad. POR- 15 is great stuff, but not perfect. As i understand it, the UV problem is cosmetic rather than rust preventive. In other words, sun makes it look lousy without affecting the rust stopping quality of POR-15. That said, i have some 2 year old exposed spots of POR-15 not topcoated that have rust in spots. I'm not clear whether you have a big major rerusting problem or some spots here and there. Resandblasting seems like a lot to me, and just abandoning POR-15 for something that may well not work like POR 15 is setting yourself up for frustration.
Also, the POR-15 folks say that while the stuff is catalyzed by water, the surface should be bone dry when you put it on. That could be where the local humidity might be a problem if there was any condensation on the chassis. I might try sanding the rusty spots a bit, hitting them cautiously with a heat gun to dry it well, then try more POR -15 and see what happens
That is my two cents.
