Rust sucks; that's really unfortunate that is all recent.
I had poor luck with the gas tank sealer from POR because it was hard as hell to use in a car-sized gas tank. I think it'd be fine on a motorcycle tank but I'd never attempt it again on a car. That crap has been flaking off for years...
As far as the regular POR, you only have to top coat to prevent UV exposure. If the spots you're covering won't see sunlight it won't turn chalky, nor will it affect its performance if it does turn chalky. Their site specifically calls out this scenario.
If you're going to top-coat, you should use their tie-coat primer. Also, other words of advice; use all of their chemicals (marine clean and acid etch). If you try to clean with your own chemicals and don't do it right you can only expect so much. Also, make sure you have everything cleaned down to bare metal either by sand blasting or wire-wheeling. If parts of your application go on top of paint you can't expect it to hold any better than the paint below, and it won't do the same in terms of rust protection. I've used the stuff for years and have sandblasted every underside part of my Datsun and coated with POR15. Let me know if you have questions.