I also use the rust reformer to touch up the rust spots, and then, I hit those spots with Rustoleum Pro Grade undercoating. Boxed frames usually rust through from the inside since they hold moisture and they can’t be re-coated inside. A friend recently had to park his 2000 Silverado due to the frame rusting through from the inside. He had coated the frame with POR15 several years ago. After researching different fluids to prevent rust, I decided to use ATF to coat the inside of my frame and cross members. It’s thin enough to spray without thinning and it appears to contain corrosion inhibitors. From the places it got on the outside of the frame, it seems to wick and spread fairly well which means it should do a good job of coating places not directly sprayed. I used an old bottle of Dexron III that I wasn’t going to use anyway.
To spray the ATF, I used a rechargeable sprayer that is pressurized with compressed air. I used a 2 ft spray tube that came with the cans of cavity wax that I sprayed inside my doors, rockers, fenders, tailgate. There are enough holes in the frame that I believe the entire frame is coated inside. The ATF seems to be friendly to the factory coating as I have seen no evidence of it pealing or softening as I’ve heard mentioned from folks using Fluid Film. Time will tell if my small investment in time and money was worth it as I plan to keep this truck for a while.