Looking for opinions and/or experience with either. Son has a truck he's planning on hanging onto - so he's looking to protect the underside, and whatever else it can reach - rocker panels, etc.
I’ve used Fluid Film on the underside of my last Jeep. It has a bit of a following among some Wrangler owners. Depending on where you live (weather) and how you use the vehicle you might have to reapply a bit more often. Stuff stays on pretty well and does the trick if you can keep up with it.
I have not used fluid film as a body rust preventer....but have used it in a few other places. This stuff is amazing....and I have read enough reports to know that is is a great undercoating....if you keep a film on your vehicle. I did not have a rust situation (AZ) or I may have used it to do an undercoating number. Everything I know about Fluid Film is good.