I have been restoring old school hot rods and biulding old jeeps for about 15 years and after the first few builds I always could see rust in the frame but the outside looked grate lol after snaping a custom jeep frame!! I had to figure out away to protect the inside as well as the out side, so I took whats called a Shoots gun (they can be had for about $15 ) they are for spraying undercoat and bed liner etc.. the thick @!*%e. well take 12' of copper line that fits over the shoots gun nozzle and hose clamp or silver solder it on wichever works for you, copper is easy to bend without kinking easy anyways crimp the end (opposite the gun) and drill about 10 holes 1/8th bit, criss cross what you have now is a rod that can be fed down the frame from any hole in all directions, it will spray everywere and in every direction making a huge mess wich is what you want inside your frame. I got a Gal of POR15 and a Gal of PRO FORM wax based rust kill/cover protector (good stuff) it is easy to shoot and soaks in well, it is also black when dry and, hard as nails so even when it comes outta every nook and cranny in the frame and drips on the POR15 it wont show cuz there both black. I will take some Video and post a link to you tube so you can see. I am also painting my bumpers, bush gard and tire rack with Gloss Black so I will top coat the hole underside with Omni as well. I am not just doing the frame I am doing the hole underside and rear diff. Might as well I got 12 gal of auto paint lol
I just dont wanna replace a fram on this thing.