One last thing - Preserving your XF by not driving in the winter will go a lot further with a simple rust spray - not undercoat. Use Krown rust control. It's relatively inexpensive and does a fantastic job. Even if your car is parked, it's still subject to humidity and condensation. Nothing can completely stop rust, but Krown protects it, lubricates the hinges, etc., protects the wiring connections and comes the closest to stopping the rust as anything that I've seen.
And for those of you that think that consistently washing the underside is good, it's actually one of the worse things that you can do. When it's below freezing outside there's nothing really happening. But if you continually try to wash the underside, you will be constantly reactivating the salt and corrosion process. The worst rusting actually occurs in the spring when we expose our cars to the freeze/thaw cycles. So spray it with Krown. It will soak into the seams and cracks where rust can take hold and you can't wash the salt away.
That's my commercial.