In the northeast undercoating is a pretty normal thing, 3M makes a rattle can undercoating that I've heard is pretty good. I had my previous truck undercoated with "white grease" It worked pretty well but wears off of the wheel wells pretty fast, the cost was around $150. Some undercoatings are like a kind of rubbery paint(Tuff Coat), I had my old GMC undercoated in this way, looks awsome but will start to peal after a year or two, your talking close to $300. Another undercoating I had done was with this wax stuff (don't recall the name, can find out if interested), it had a yellowish color to it and didn't offer much coverage, cost $80. There is this out fit in central Vermont, Cararras hot-oil undercoating, that does oil undercoatings, they spay oil in every nook and cranny of the vehicle, it's messy but works very well and is fairly cheap (about $45 for a cruiser, $55 for 3/4 ton + trucks). You typically want to get two of the hot oil undercoatings a year, they really are very affective and tons (seriously tons) of people get there rigs done this way in Vermont. One could probably rig up a way to do this kind of thing at home in their own shop. Hope you can get something from this.
Does aluminum need undercoating? ??? Mine was sandblasted, primed with industrial grade generic epoxy primer (5 coats) and then painted with flat black auto paint (3 coats).  urable, sheds oil, dust, mud etc easily, and not too heavy on the wallet. Especially for me cause the PO did it in his own shop (Owns industrial coatings buisness) If it was me, I would sandblast, (not for new aluminum tub), either try to find what my PO used, or use POR, or Zero Rust, or maybe that 3m rubberized stuff. That might get expensive since its in a rattle can though. Would definatly NOT use bedliner of any kind. Maybe someone that knows more about aluminum can tell us if its necessary to undercoat it? :dunno:
3M Rust Fighter I should be considered in addition to exterior rubberized type coatings. The real rust/corrosion starts deep inside the seams and crevices... this stuff remains tacky and flowable. That last word is important... is stays viscous and continues to flow and coat veerrrry slowly over the years. Spray it into cracks and such from above and it flows down in.