I'm not even sure that the assumption all wood exchanges moister is actually correct. Take the Mary Rose bows found underwater. I believe some have even been shot! Also finishing oil from liberon makes this claim "A most dramatic demonstration of the qualities of this finish was given by Jan Sanders who, having turned a small oak bowl, applied three coats of finishing oil, allowed it to dry and then poured a quantity of blue water based wood dye into the bowl. After gently swilling it around, Jan poured the dye out and wiped the bowl dry. Needless to say no trace of the dye could be detected on the inside of the bowl. This oil will give good resistance against water, alcohol and food acid marks and will give a matt to satin finish dependent upon the number of coats. A gloss can be achieved by applying a paste wax. Also approved safe for toys. Coverage: 1litre to 8 - 12mē. Drying: approximately 5 hours."
My feeling is that modern finishes DO seal a bow almost completely, if applied correctly. And I for one like this, if my canoe capsizes I want to know that my bow will be fine after a little dunk. Some of the new floor finishes are even more bomb proof!
There is one very big problem with that test you described. It deals with liquid water, not water vapour. Liquids can not diffuse through solids, but vapors can, regardless of how well sealed the solid may be.
If you have a perfectly sealed tank of propane (no leaks), and you leave it for long enough, that propane will diffuse right through the steel walls of the container. All there needs to be is a concentration gradient, so as long as the concentration of propane outside is lower than the concentration of propane inside, the gaseous propane will diffuse through.
The same thing happens with bows, and water vapour. It takes time, but it does happen, and there is no finish that you can put on a bow that will seal it nearly as well as a propane tank so it happens much more rapidly in bows.
Moisture will move in and out of a bow regardless of how well it is sealed. If the bow is dryer than the air, moisture will diffuse in, if it is wetter than the air, moisture will diffuse out. You can slow it down, but you can't stop it.
Summary: You can stop liquid water from soaking in, but not water vapor from diffusing in.
Knowing that, I can't agree that modern finishes cause bows to dry out. They might let in moisture slower than primitive finishes, but they would also release it slower. Both kinds of finish will maintain some sort of equilibrium with their environment.
