Rust inside tube.

Maybe I just worry too much about stuff but what can be done to stop the rust inside of a tube structure?
On the backhalf for my truck it will be completely closed other then four bolt holes for the fuel tank straps. I was thinking of pouring POR15 or something similar in one of the holes then flipping it around the yard to coat the tube, then draining any excess.

Would this be a waste of time?
Is there a product designed for this?
Your thoughts?

I know they sell a gas tank sealant that is used in a similar fashion. I believe it's called Kreme. POR15 or a good tractor paint would probably work as well.

A lot of times, the inside of steel can be pretty oily, so you would probably have to pour something in first to degrease everything so that the paint would stick. You may want to throw a bunch of small rocks in as well to help clean the walls.

If you could seal up the 4 holes for the gas tank, then I wouldn't worry about. Look how long air tanks last, and they are constantly having new moist air pumped into them.

Couple of techniques I've used:
<>Rust inhibitor is sold in spray cans.... saturate inside thoroughly from both ends of tubing and let it drip out. (will cause fumes if you weld after application)
 <>Before you weld pieces together, you can spray the inside w/ cold galvanizing spray... which will protect the welds on the inside after you lay bead.
<>You can spray existing rust w/ a rust encapsulator.

They used to treat the tubing at Williams with some kind of coating. Ill ask dad if he recalls what it was. If I recall it foamed and then dried as a coating...