Back when I started all this, my plan was to simply oil up the frame and call it good. Well, since I pulled both tanks and everything was easy to get at...well... you know...
Thought about using POR15 but ended up going with a product call Corroseal. Much lower cost and its a one stage process. Reading up on POR15, it seems many get good results with sandblasting or other fairly involving prep. Normally I wouldn't trust an easy fix like this, but my Dad used it on some tractor rims many years ago. Solid layer of hard crusty rust but darned if it didn't hold all this time. He ended up topcoating with some cheap industrial enamel but still no rust bled through after all this time.
I still did some prep work using a wet sandblasting attachment on the pressure washer, lots of wire-wheeling and good of fashioned bashing to get the thicker crust out. The real challenge was scraping the old undercoating off. Whatever the PO had put on, it was good quality stuff.
I was also a little concerned with the cross member that supports the rear of the mid-ship tank. Rust flakes were collecting between the joints and I wanted to make sure it wasn't eating into the main C-channel. Turns out it wasn't but I ended up replacing the cross member with the one from the donor truck since it was in better shape.
Long story short, I blasted/prepped, dosed the rust (turns black in a few minutes), then inspected and retouched the rust treatment the next day. After that, I applied a rubberized undercoating to everything rear of the cab. I've used similar undercoating in the past and had good results so I hope this holds. Supposedly it has cellulose fibers to prevent cracking over time.
Next up, fuel tanks from the donor truck will be going in. They have minor surface rust in a few spots but nothing major. Both will get fully undercoated before getting strapped back in.
Oh and the pictures don't really do justice to how messy a job this is...yuck!
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