I did a frame up pickup resto once. Had everything dipped. Frame got PR'd. That was a waste of time and money. Even dipped, you just can't seal the thing up enough to keep it from peeling. The only way to keep it from rusting is.... Well,......
So, I'm going to say something (some will) think is real stupid here. ANd sorry, this only applies if it is not a show truck, where the only goal is to have it look neat, smooth, black and avoid rusting.
What I have been doing is really, really cheap, but it actually looks great. And, I've never seen it peel because there is nothing really to peel. Geez... I don't know if I should even say it...... I know I will get ripped on, but here goes!
I just knock down all the rust in the normal ways, mostly wire brushing, a little hand work. Knocking off good tight paint doesn't make sense.
Here we go. >BLACK PRIMER<..... soaked with fluid film yearly. But primer is porous! Yes, perfect ! The fluid film permeates to the frame always and without melting the color. The fluid film never resides on TOP of the paint, but IN the paint. Frankly, it looks really nice and has become my go to on any non-show car scenario. Its the bottom of the dang truck and all I care is that it stays in good shape.
Touch up is amazingly easy too. Those few areas that get water blasted off and surface rust appears just get wiped off, wire brushed, black primered and Fluid Filmed again. Everyone looks at it and thinks I did a major frame paint job. NOT!