fluid film or oil works - traditional undercoating does not.
I've never used fluid film but know folks that have and it seems to be every bit as good as oil however both require resprays although I don't think the fluid film needs it quite as often. That said, I use gear oil to protect my truck - has to be heated to apply and get a good spray pattern. It holds up well and the under side of my truck looks as good as the day I bought it.
While modern day vehicles are far better at resisting corrosion - they still will eventually succumb to it if nothing is done. The really critical parts are those you cannot see such as inside the frame rails on trucks. Many rot from the inside out. Using oil it will bleed and spread especially in warm weather, which allows it to penetrate those problem areas.
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