I'm working on patching up the rust on the roof area of my cab. I want to spray something in all the cavities that will convert the rust or stop it. I've looked at the Eastwood internal frame coating that has a flexible tube i can spray with. Also see KBS cavity wax. Any thoughts on what would be best for this? I think something kinda runny would be best so it gets into all the seams and such

 

They sell an undercoating gun kit that comes with a flex hose with a 360 nozzle sprayer you can use.
I'm not sure what the eastwood one looks like, might be the same thing I just described.

 

My theory mind you. I think the main reason for rust in the interior of the cab is fed by condensation of warm heated cabin air against a cold interior surface of a roof, assuming no leaks. If defrosting the windshield of interior moisture like breath, that moisture is in the cabin air then. When my AC was working, I had a switch to turn on the compressor when running the defrost, made for fast clearing and that air went through the evap core if the recirc door was opened. I no longer have operational AC, so I'm back to just plain heated air ... not heated dry air.

My '77 recently got a good going over, body panels, paint, etc. We had the glass all out and in putting new rubber in, we found a few little signs of rust above the rear glass and Roy rebuilt part of the lip, but the area above the windshield and doors were all fine. I'm gonna maybe drop the "velvety like material on cardboard" headliner, and clean any loose rust off the metal, then carefully brush in a complete coating of Iron Armor bed liner. It will maybe prevent moisture reaching the metal I think. Then I'll put the head liner back. A buddy suggested I put a sheet of thick plastic between the head liner and roof as a vapor barrier, but I'm thinking the Iron Armor will serve as a moisture barrier.

I meant to do it this past season, but it went from hot drought to winter overnight.