My personal preference is to strip paint from cylinders if there is any indication of bubbling. Then the metal surface is treated appropriately (oxidation removed, ensure that there are no pits greater than PSI tolerances, etc.). If all is good, I leave aluminum cylinder bare. I cold spray the old steel 72s with a thin coat of galvanizing primer and then a top coat (I usually use Rustoleum paints). I do not recommend bedliner paint since it gives you a false sense of security. My previous truck developed significant rusting under the bedliner spray where salt water penetrated tiny pinholes not apparent to inspection by eye. The salt water was thus trapped between the paint and the steel, and the bed rusted. On my current truck I left the bed as it came, always hose it down after a dive trip, and use a rubber mat to protect the bed (I picked up a couple from Lowe's for about ten bucks each).