Hi, JayBees - Did you ever get an answer to this? I'm going to need to rebuild the sidewalls of our overcab bed, and I, too, am wondering about that outer layer of luan (next to the Fiberglass) and whether it is necessary. AND if it CAN be replaced, does the replacement substrate NEED to be glued to the fiberglass? I'm looking into a product called GoBoard and a plastic wall panel on the inside - and wondering if that could replace the rigid insulation and exterior luan (and NOT be glued to the exterior fiberglass). Thanks, in advance, for any advice and experience anyone can offer.