It depends on your budget, and how thorough you want to be. Aluminum or steel, I would highly recommend an etching primer, one that is a two part using the primer and an acid reducer. If not an etch primer, than a conversion coating followed by a fleet grade epoxy, look for the ASTM salt spray standard of 500+ hrs. Let the epoxy cure and then scuff it until you are sure it's overkill with red scotch brite pads and apply the bedliner. Almost any of the liners can be thinned with 5% or so of acetone, this will give it better flow into seams, etc.
It can get expensive, but a two part seam sealer before lining is a good idea too. My goal is to have my cruiser laugh at rust the same way it does mud.