I think I read it in one of your other posts, you just use waste oil and spray it down? Are you misting it on and just letting it sit for awhile?
I use new oil (I get it free) and mix it with a little diesel. I do the inside of the bed, bed rails, doors, cab corners, rockers, frame, inside fenders and under cab.
I use the used oil out of my cars on my explorer in the same fashion, just soak it down and I try to drive where it’s dusty shortly after to help it stick longer. I steam genie everything back off twice a year, touch up paint and recoat so it doesn’t get so thick and nasty but the thick and nasty protects better, my friend has a 77’ Chevy has grandpa bought new and coated it every oil change, it was coated like gel but when we steamed the underneath and cab corners it still looked new, where most are junk around here.