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On the outside as in when you open the door?? Not on the visible exterior right?
Inside the doors - drilled holes in the doors from the jamb area, snake the hose in there from the aerosol can and spray as you're drawing the hose back out.
Inside the rocker panels, the entire bottom sode of the truck, up as far as I could get inside the bed (from the bottom), inside the tailgate bottom, suspension, frame, driveshafts, transfer case - pretty much everything that was visible and a lot that wasn't...
I did not spray the door jamb area - never seen a problem there witha vehicle that's kept reasonably clean..
I've used the BB system since 1986...Cheap and effective. I use the BB gun (cheap as well) for spraying oil or bar and chain oil and it sprays Monsta bedliner,Gator bed liners over the years.
I will have to try Fluid film but it's pricey. Bar and Chain oil is cheap and it only takes a few mintutes to spray. I drill holes in the side bed walls and use the BB plugs and drill bit to do this. No big deal. When I sell the buyer likes it was undercoated professional like.
Choice is yours. Use old engine oil,BB system stuff,bar and chain oil, or fluid film. Depends on your pocket book. But one thing is for sure spray it. NH starts salting in October and goes into April and May..