This sounds very interesting in regards to strength and weight. Your absolutely right about the sanding! Lot of time to sand, not to mention prime and paint if you want a finished look.A note to anyone considering wet layup, it can be done affordably (epoxy FG over hardware store XPS foam), and it isn't that hard. Should be more efficient than gluing on sheets (strength/weight). You can also more easily do curves. Reinforcing edges and corners is also easy, just add more layers.
But if you want a really nice smooth even surface you'll kill a lot of time trying to acheive it. I knew this from the start, and I'm not going to try. I'm planning a final texture coat to sorta make it look ok.
Do you have a link to this product or pics of what your doing? My only thoughts/concerns with wet layup is flexibility. Could potentially crack.