When I used FF on my tundra in late oct., by march, only the most protected areas still had some FF left....all other steel chassis areas were showing surface rust. I use it on my tractor, mower, and snowblower decks which don't see heavy rain, or wet slushy roads.
Seeing that I was doing the same job over, and over every year, I had to find something else longer lasting. I did a bunch of reading, and found bmw, and mercedes both ship their cars with chassis's coated with cosmoline. Figured if good enough for them, it's more than good enough for my lowly taco.