Various mechanisms exist by which moisture will sneak into a paper hull. It need not be through cracks and crevices, (but we'll assume you've done a perfect job building your paper wonder and have none).
Moisture will eventually find it's way through even an unbroken layer of paint or other surface coating. According to our authoritative source (Modern Surface Coatings, by Nylen & Sunderland, Interscience Publishers, 1965; (OK, it isn't that modern, but I doubt that the general laws of science have changed that much in the interim)), the process is governed by Flick's Law:
p1, p2 are water vapor pressures on either side of the film,
A is the area of the film,
t is the thickness of the film,
K is a coefficient of permeability, and
W is the amount of water passing through the film of paint.
The value of K is determined by the properties of the paint while t will be a function of how thick you slop it on. The rest is mother nature's doing. Our source provides some examples of how much water would pass through a 30 micro-meter layer of paint in contact with water (back in 1960) :
|Latex||about 25 mg/cm2/day|
|Linseed||2 to 2.5 mg/cm2/day|
|Alkyd||1.1 to 1.3 mg/cm2/day|
Most synthetic resin based paints (e.g. single part epoxy paints,) have values similar to alkyd base paints. Two-part epoxy & polyester resins have significantly lower values of K and can be considered to be true moisture barriers. In addition the two-part resins usually are applied as thicker films than paints and varnishes, (i.e. a larger value of t for Mr. Flick's law,) which further enhances their waterproofing properties.
Thus oil-based paint used as a protective coating will allow about 10x less moisture into a paper hull than a latex paint. The reverse is also true so an oil paint will tend to stabilize the moisture content of the hull, allowing it to change only slowly with time. This does not, of course, apply to the seepage of water or moisture through cracks, joints, & etc. So for durability and long life for your precious paper hull, sand it as smooth as possible and then coat with a high quality synthetic resin-based coating.