We used to use Fluid Film pretty extensivly on the not-often-opened areas of our helos in the Navy, those things get regularly sprayed with salt water mist and live outside next to the beach. It does wonders really, very rarely have I seen corrosion inside those spaces, when I do it's usually because someone didn't apply it or didn't apply it correctly. We've since moved on to something called Cor-Ban 35 for most of those areas. It's more of a hard coating, it'll chip but it's a ***** to actually remove, seems pretty tough in my limited experience with it. Had to remove some on a large panel and I really could only do it with a wire brush and loads of solvent to try and break it down, don't think the solvent did anything to help but kept the dust down at least. I know FF works great, but it being just a soft coating, it rubs off pretty easy so you'll be reapplying regularly. I've heard lots of horror stories with the rubber like undercoating systems, seems cuts from kicked up rocks making pockets for salt to hang out is the general consensus. I couldn't tell ya if Cor-Ban would do any better besides it not being soft so it can't make a pocket, I'm sure it would still chip and require touch ups every year. It's also around 72 dollars a quart on various online sources and I would think you'd need a gallon minimum for the full underside of the truck, probably more as I seem to remember the rattle cans not lasting very long, but it's also an aviation product and like motorcycle products, it's expensive as hell for no real reason other than it being for a relatively small market.
Kinda makes me glad I live in California again, don't really need to worry about rust anymore.