should you rust proof a used vehicle?
Nope. You should not even rustproof a new vehicle.
Today’s cars are manufactured using alloys, conformal coatings, bonding technologies, joinery, drainage, and ventilation designed to inhibit rust. Rustproofing may breech the protections (in application) or plug the drains, promoting rust.
I wouldn’t do it. In fact, if the rust proofing isn’t done properly, it may accelerate the rust since moisture can become trappped under the rust proofing. My first wife’s father bought a new Ford F-100 and had it rust proofed by a company that did a lot of advertising. This vehicle rusted worse than any vehicle I have ever seen and it was “rustproofed” as a new vehicle. I think the situation may be worse on a used vehicle.
Pickup? Look at the frame rails and see if there is a wax-like coating on them. If there is one, rustproofing won’t do much additional good. Fixing holes in pickup bodies won’t be too difficult when they eventually happen. Save the money and put it toward body work if you own the truck for that long.
Rustproofing does no good if you sell the truck two years from now. One major accident and you will want to have a different truck.