Blistering of Finish

I'm looking at purchasing an LS-3 but the owner has
noted that the Urethane finish has blistered in several
places on the tops of the wings (The glider was re-finished
in 1996). According to the owner they are very small
blisters which have left pin sized holes. I'm assuming
that this would be caused by mositure which has made
it's way below the paint? He says the trailer is dry
and has a soler vent so I would assume humidity in
the air has caused the problem? and not moisture on
the surface.

What are the implications/underlying problems of the
blistering? and could I fix this easily or am I looking
at sanding down/re-finishing the entire surface of
wings again in the near future?

Any advice would be appreciated.