That's a good question Joe. You're right, nothing under the bed will ever rust enough to hurt anything in your lifetime except maybe bolts and nuts.
I was recalling my MDT days. I had a bed built out of steel much lighter than you are using. In that MDT I could hear rocks from the tires being thrown up against the metal bed. That's when I decided to undercoat it. You won't hear that in BART due to heaver steel, and better sound proofing in your Volvo.
I would have the exposed outer sides and side boxes painted ASAP though, because unpainted, they will rust unbelievably fast. You don't want to paint it a month later and paint over rust. Thats what I did, and the rust started coming through the paint in less than a year. Same thing applies to the top of the bed too unless you are going to use Rhino Lining or something like that.
Larry
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