In an off-line discussion, the topic of covering getting loose when it's damp outside and screwing up the airplane trim (and maybe the airflow) came up. Part of the discussion was about different types of covering and how they react - in the example case, the airplane used "doctor's paper" with what might be described as an "acceptable" weight-saving dope finish - and not to be uncharitable, good enough to not stand way out, but also, no threat to Windy's concours award. In several similar cases of morning fog/dew/condensation, the airplane performance degenerated dramatically, only to recover once it dried out. The underlying structure is a conventional D-tube with open-bays and "perpendicular" ribs.
This is a very well-known phenomenon and the reason that FF guys, and most stunt fliers, use synthetic materials instead of paper products for covering. But I got to wondering about how the doctor paper might be different from the real deal silkspan. So I put my little USB microscope to work again:
Same lighting conditions and same magnification (about 150x). As you can see, the doctor's paper is definitely NOT the same as silkspan, it is amorphous short-fiber pulp made like classic wrapping/packing tissue, where as the 00 Silkspan (probably K&S, maybe from some other distributor that used to exist but I forget) is the traditional long-fiber strongly-linear-grained arrangement.
I also just hand-tested the wet strength, and even though the silkspan is only the lightest normal OO strength, and the doctor's table paper is pretty solid and heavy, the silkspan had much better wet strength.
So, I would not say the doctor's exam paper is unusable for our purposes, obviously, some pretty good airplanes have been made with it. But I am now not too surprised that our anonymous modeler had some trouble with it in damp conditions, the lack of directionality alone would seem to greatly alter the structural deflection, and with more pulp, I would expect to absorb more water.
For my purposes, I think I will stick with polyspan, which more-or-less doesn't respond to water, for an open-bay structure. I think it doesn't make any difference over a solid surface, all you have to do is span the cells in the balsa.
Anyway, food for thought.
Brett