I am getting ready to replace the cab corners on my truck.....I have one question after getting the replacement corners welded on how would you rust proof the back side of the corner (no access)

 

On my 79, inside the door jam, right where the lower body line bulges out, there is a 1/2" hole, with a black plastic plug in it. This was there from the factory. Pop the plug out if your truck has one, or drill a hole in that area. Stick the end of your undercoating gun in the hole & go to town. I used to spray motor oil once a year in my cab corners (among other places) back when I drove my truck year round. Cab corners never rusted in 190,000 miles of Pennsylvania winters.

Also, I like to use a metal prep on the front & rear of any replacement sheet metal before I install it. Something with phosphoric acid in it. Next I paint the rear of the panel & the inside area of the cab corner with Rustoleum rusty metal primer. Then I weld it in, while the paint is still wet. It will smoke some, & there may be a small fire. Mostly though the paint seems to hold up to the heat. Then a coating of gooey undercoating, & an oil bath once a year.

 

bare metal weld primer,I think by SEM,,,,I followed up by using eastwood extended wand 12515Z Internal Frame Coat - Internal frame coating
Eastwood Internal Frame Coating 14oz Aerosol