Early in the development of S&G hulls very few designers specified glassing entire hulls. I built the old Chris Kulczycki design called "Yare". Mine weighs about 32 lb. minus hardware. To keep weight that low it had to be built from 3mm okoume and no glass sheathing could be included. The compromise is the hull is limited to fairly deep water.
Three millimeter plywood was required because the building method was "tortured plywood". The ply is almost rolled into a tube so the hull comes out fairly stiff. I don't know if 3mm ply would work as well with a 4 panel hull.

: do you have to put fibreglass sheet on the hull (inside and out) or
: can you just tape the seams and use a paint to seal the hull ...
: how thin can the plywood be if you can build a kayak like this (if
: it's possible). cheers