Just wondering if anyone has ever used this Fluid Film: FLUID FILM | Powerful Corrosion Protection & Lubrication (fluid-film.com) on their frames & undercarriage for winter use to help it not rust out?

 

I think you'll find that several on here do/have. If they don't chime in (some have disappeared...) try a search maybe.

It's an annual ritual for some plowers and it's used a lot to spray into cavities etc. From what I recollect, it needs to be reapplied regularlyish.

 

I have used Fluid Film on many of my rigs.
Fluid Film doesn't last as long as some oil spray on applications I use, on places that see a lots of water and weather. I like to use the fluid film sprayed into my door panels seams, above the windshield where they like to rot out. To get it above the windshield to the metal that needs it, I spray it through the sun shade visor holes and upper trim screw holes till it runs out the seams. I do this every few years and it last well there.

On the frame for the long rough winter up here, I actually spray on Bar and Chain oil, thinned down with a little diesel to help it soak into the nooks and crannies.
On my winter rig, it is a yearly chore.

 

I have been using fluid film for many years, but have recently noticed one draw back to using it under the vehicle...it eventually kind of "cakes up" and gets covered in dust and grime, and this year I noticed that it was flaking off in areas and under it there was rust starting. So I have switched to paint brushing oil on the under side of the truck, and using the fluid film inside and under the hood.

 

I don't use under the whole truck (too $$$ for me) but I do spot-spray it on key areas and bolts/fittings I want to remain serviceable.