I have a powdercoated frame on one of my Malibus, and on one of the other ones, I did POR15 on the entire floorpan, after removing any rust that was there, then using their metal prep. Yes, it's shiny, but I wanted the protection this stuff gives against RUST. It's going to be under carpet anyway. If you prep the base material correctly, it will last a lifetime. I did my parent's steel mailbox post probably 10 years ago or so with POR15. Lightly topcoated with Rustoleum semi-gloss black afterwards (after a light sanding per the instructions). Still looks like I just did it. Wisconsin winters with all the salt by the end of the road, too, splashing it all the time, and getting rained on etc...
I'll use it again, but I like powdercoating on chassis parts and my frame. If you do all the labor taking the body and suspension parts off the frame, just have it powdercoated. Touch up any rock chips with brushable Rustoleum. Thing is, you won't get many chips, because it's tougher than paint anyway...