Truck bed-type surfaces such as full exterior jeep bedliner, full exterior truck bedliner, or any surface with automotive paint needs no general sanding if the paint is in good condition. Light scuffing is always advised to dull the surface, and it is recommended to clean any oils or other contaminants with acetone or lacquer thinner first.
Fiberglass and unpainted metal surfaces are recommended to be primed before application in general, although steel can sometimes be successfully coated with only scuffing. Primers are always recommended on non-porous surfaces which have no current auto or marine grade coatings applied. Any automotive grade one to one epoxy primer or other high quality primer will suffice for most metals, but PPG Spectracom 701-702 is generally recommended for bare aluminum. In the case of fiberglass, out-gassing can sometimes be an issue if the material has not fully cured yet, so appropriate primer is recommended.
Wood, cementatious substrate, all stone, et cetera require a clean surface which is free of oils or loose materials, but generally do not require a primer. It is often advisable to apply a light coating first and let it cure for at least 1 hour before applying additional coats in order to establish a solid base for the full build application.