I currently have my hunting rig sprayed with Fluid Film, the only thing I don't like about it is, FF collects dust, crud and whatever else will stick to it.
I would avoid the rubberized undercoating like it was the plague.
From my research and if there was one close, I'd go with
Krown Rust Control. passport makes a valid point above about keeping the under-carriage flushed, but bare metal still tends to rust. My 3 year old Tahoe is proof of that. I plan on undercoating it with a mixture of diesel and oil.
Best of luck.
