I believe that
: glassing plywood is often unecessary. We built plywood boats for 40 years
: before epoxy coated glas became fashionable. Glassing adds a lot of weight
: for a only moderate gain in stiffness.

Brian, 4mm ply in a little kayak that is designed for beach launching/landings and waves will not be sealed and/or rigid enough with glass only on one side, especially the bottom of the hull. Big difference between a 20' skiff made with doubled layers of 3/8" play and a kayak with 4mm ply where a 200lb person will step into the boat, same with a kayak that is dragged one across the other as in rescues for the deck. Certainly with some panel sizes and tape on the interior the flexing can be minimal but I would be surpised if a canoe made with 4mm (is that what Patsys canoe has?) would stay sealed with out glass on the interior if the panels can flex inbetween the tape.