recommendations for paint/sealing of the bottom of the unibody?

I used herculiner (rubberized truck bed liner) on the underside along with my passenger compartment and am VERY happy with the results. I would do it again in a heartbeat.
Best part aside from the results, is that it didn't take any special tools or experience to apply. Just a roller they provide with the kit and a foam brush for the hard to get areas. Takes 2 coats to get proper coverage but it goes a long way.

I don't have a good pic of the underside, but the results were the same. I also did my truck, inside of my front fenders and the firewall (passenger compartment side).

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