After reading some very favorable posts on here about it, I'm gonna get a couple gallons and coat not only the underside of my 97, F350 but my wife's 2010 Transit and my 2015 Focus.

I have access to a drive over, walk down pit at the shop so I can get underneath easily. My question is what should I apply it with?

I was thinking of buying a Harbor Freight spray gun with small pressure pot for pressure delivery and spraying it on with that.

We live about a mile off the pavement on a dirt road and no matter how slow I drive and how often I clean the undersides, the stone / dirt road surface 'sandblasts' the undersides of my vehicles.

Hopefully, the Fluid Film will retard that a bit.

 

I don't think Fluid Film will hold up under your conditions, not without it being re-applied frequently. Honestly I'd suggest some sort of appealing mud flaps to minimize the splash back.

You could also look into making your own wheel well inner covers, something to isolate the tires from the underside of the body---this too would keep underneath pretty clean and not prone to rock chips etc.

Fluid Film is a great product but not for everything.