Sorry I don't have a definitive answer right now, but recently a cardboard bicycle was in the news and it is waterproofed- if anyone can find out what that process is.
And a while back I posted about a cardboard and plywood car students built- perhaps there is info online about that somewhere.
http://ecomodder.com/forum/showthrea...ign-22627.html
Google "waterproofing cardboard"- there's plenty of hits, including this:
http://ecomodder.com/forum/showthrea...oard-4455.html
Also I wonder how well cardboard holds up if it's soaked with polyester or epoxy resin as commonly used in fiberglass projects.