Measuring tools and instruments often come in wooden cases. I've seen
mahogany, birch, poplar, pine, and several other woods used. The idea
is that the case will absorb moisture from the tool and thereby keep
it from rusting. Usually, these boxes are finished with a varnish on
the outside, and less often, on the inside. When the inside is not
varnished, it is usually left bare. I'm reasonably sure the original
finish is a varnish, because I've tested for shellac with denatured
alcohol, and these boxes generally predate polyurethane.

When I buy a used instrument the box is often in rough condition, and
I like to put them back in good shape by regluing, sanding, etc. I've
been leaving the inside sanded but unfinished (no coating of any kind)
and spraying polyurethane on the outside.

1) to maximize the water absorbtion, should the inside of the boxes
be left unfinished?
2) to maximize the water absorbtion, should the outside of the boxes
be left unfinished?
3) Is clear gloss sprayed polyurethane a good choice for finishing,
given that the water absorbtion issue is much more important than
protecting the finish of the box?
4) For making a new box from scratch, what wood choice would maximize
the water absorbtion?
5) I've seen some pretty old tools with ground cast iron faces arrive
inside intact boxes without rust, so I'm kind of tempted to believe
the whole water absorbtion idea. If anyone can substantiate this
theory, or offer an alternative hypothesis, I'd be very grateful.

I've cross-posted to our brethern over in rec.woodworking since they
have specialized knowledge of woods, as well as a shared deeply-held
hatred of rust on tools!

Thanks very much!
Dave