I believe AMS-Fligth in Slovenia is building gliders with only PU. They
have developed a method for spraying the PU in the mold prior to layup so
that the pieces comeout with a PU finish. This is the way my APIS was
built. The finish is all PU, no gelcoat that I am aware of.

http://www.ams-flight.si/ abour 1/3 of the way down the
page "AMS Sets The Standard In Glider Finishes".

John Scott