I'm having a debate with my co-builder. We've done the deck, and cleaned up the underside. We're debating on if we should glass that, or just the top. The regular Mill Creek (16.5) just glasses the top. The Shearwater manual that you're sent to for the deck does glass the underside.
Of course, the kit only includes enough material to do the top, which is why I'm in the "seal with epoxy, glass the top only" camp.