I have never used FF as an undercoating product except maybe in a few spots here and there when I was under my truck/car spraying some dry spots. (I normally have some spray cans of Rust Check or Krown on hand)
Luckily for me, Krown and Rust Check shops are readily available where I live and I usually have my vehicles rust sprayed annually or semi annually depending on how harsh the winter was before?
Regardless, and like I said in another thread, when it is mild out in the winter and you have the means, spray underneath gently or semi gently with your garden hose getting up under the fenders, bumpers, undercarriage, etc, to spray the dirt/sand and salt out.
In the spring I do this every year really thoroughly making sure I spray inside bumpers, rear and front diffs, under the lower lips of the fenders, etc, basically going everywhere, until the dirt/sand is no longer visible and the water starts to run out clean again.
You'll be amazed the amount or dirt/sand and salt that runs out of these areas.
If I didn't have those shops so readily handy, then I would most likely also use FF or something similar to do it myself.
Anything is better than doing nothing at all.