I did a friend's CJ underbody & frame with POR 15 & plain old paint about 5 years ago. For my XJ I used rattlecan rubberized undercoating that I touch up pretty much any time I get any scratches from rocks. So far neither vehicle has rust underneath. They both got flooded & sat in nasty salt water following Hurricane Sandy, and both sat without being hosed off underneath for weeks following the storm (house damage was a bigger priority).
At this time both vehicles are as they were before the dunking. Can't speak to inside the framerails, but visually the undercoating has held up very, very well.
1992. XJ. Built.
Red Jeep Club Member #425 "Mud is evil."