Your car is only as good as the frames it's built on.
That is why I recommended getting the Rust fighter inside the frames.
It consists of a shutz gun on I bought on e bag for 20 something bucks, the wand is like 10-18 bucks and the Rust fighter is about 20+ at your local auto paint body store. I also retro fitted the gun tip with a short piece of hard plastic hose, a fitting, and a few hose clamps. Tighten the heck outta those hose clamps.
After I sprayed it inside the frames, I took an air gun nozzle tip (the one thats a real small bent tube end) and blew it all around to spread it evenly around inside the frames. Keep your air PSI pretty low between 30-45 PSI to the shutz gun when spraying the Rust Fighter inside the frames. Pull trigger in short bursts and plug the front hole off on the end with a screw so it only comes out the side holes. This stuff is thin and it may try to blow out the hole on top on the gun when you pull the trigger. Put your finger over the hole when you pull the trigger.
I think it's an emulisfied wax and oil mixture. So it does stick well once it dries inside. I went thru almost 2 bottles and repeated the process a week apart so as to let the first coat tack up. Make sure you shake this stuff up really good and have a plastic sheet on the ground to catch drips.
I really think this is the best route to go instead of trying to thin out POR-15 or Rust Bullet. They tend to bubble up if put on too thick in areas which will happen where as the Rust fighter won't have that problem.