POR-15 is all hype. I spent a few hundred dollars in POR-15 re-doing the floor of my truck, and completely coating my bumpers. Needless to say, the POR-15 has actually flaked off in places and the bumpers have developed numerous rust spots. The $4 wal-mart rattlecan spray paint is holding up better than the POR
You did the prep to the letter on the can? A friend & I have extensive experience with POR-15 (he more so than I) and the only times that either of us has had a problem is when we did not do the prep
exactly as they say to do it. Even substituting normal DuPont Metal Etch for their "Metal Ready" has lead to inferior long term performance. I have experienced exactly what you describe and it was traceable back to my not doing a good job of prep.
POR-15 is a moisture cured Polyurethane, it doesn't want to be treated like normal paint. The longest lasting POR in our 'inventory' is the Scout II tub interior that got misted on after painting it. You can even see where rain drops hit the paint.
POR does have a problem though, it does not deal with UV exposure. If it is going to be somewhere exposed to sunlight, even just reflected sunlight, it needs to have a top coat. I have had good luck with spraying on Rustoleum when the POR has cured to the slightly tacky point.
For the interior of the tubes a climbing/crawling oil is one option. I think Boeshield is in this class. WD-40 is most definitely not. Look up this type of oil on the Aircraft Spruce page. It is sold to rust resist welded tubular aircraft frames. Never mind:
http://www.aircraftspruce.com/catalog/appages/tubeseal.php
Another option might be the "Waxoyl" that I've seen mentioned on the LR forum. Not too familiar with that stuff though, would bear more investigation.
Eastwood sells a kit to coat and seal interior spaces (like inside of rocker panels). My same friend has used it, but I nothing more about it.