Wilbur Wong
Member
I think that would be unnecessary. A darkroom shouldn't have a moist atmosphere like a bilge in a boat has. And epoxy will give enough protection from water and chemicals. Just skip fir if you want the surface to be fair later as the annual rings will always show through any kind of paint over fir plywood. Here in Sweden I would choose birch plywood and just check the ends for voids.
/matti
Birch plywood is an excellent choice for dimensional stability.