Personally, I think FF smells horrible and after using it on my former rust bucket, there is a zero percent chance I'm going to slather any other vehicles I own in that stuff. It works great, but the smell is kind of nauseating to me. Some people like it though- smells are such a weird thing. It's also nasty to work around and attracts dust.
Maybe if it was parked outside all winter and I could easily put it on a lift and pressure wash it all off in the spring, then I would consider giving the underside a FF bath. It doesn't smell as much when it's cold, although a fresh coating under the whole underside of the truck is going to reek bad for a long time, cold or not.
I have been, and plan to continue spraying any visible surface rust with a rust converter, then top coating it with a good quality spray paint.
I use this for the rust converter.
https://smile.amazon.com/gp/product/B0009LPJ0Q/
I use Rustoleum Professional flat black and buy the big cans by the 6 pk.
https://smile.amazon.com/gp/product/B01MZFKX8B/
For the rockers, door, and tailgate bottoms/pinch seams I'm planning to use something purpose made for it.
Something like this:
https://smile.amazon.com/gp/product/B00TVH7R5O/
or this:
https://www.amazon.com/dp/B0741FHDPF/?tag=fordraptorforum-20