Great product Fluid Film, used it for 7 years on my 11 FJ (New England and NYC winters -100% outdoors and it saw its share of salt)

Did a Fluid Film treatment on my 18 4R in the fall and in the spring of 19. It does need reapplying after some time. Good you can power wash it off if needed.
It will attract some dirt but I'll live with a dirty looking underside for the protection value. This fall I did a Fluid Film coating and then WoolWax application over that just before the first snow and road salting. WoolWax is the same as FluidFilm but thicker and does come in a black color option. I just did 4 rattle cans and also wiped a coating on the rims (saw a video from Australia where they swear by Lanolin oil for the 4x4's) again its not permanent but does stick well and will get a light re-coating (after a good wash) in the spring.

Good real world test here YouTube

In my experience w the FJ the more (coating- Fluid Film/WoolWax) you can keep on through the winter elements the better. Doesn't look pretty and drives me its (OCD) but its a battle against the rust here.