On Feb 22, 12:14 pm, Andrew Barss wrote:
wrote:

: I didn't understand the question. Are you meaning resistance to
: outgassing vapor transfer from uncured wood, or vapor penetration
: through the finsh to the wood?

Both, I think, though the focus of the article was on
limiting seasonal expansion in a humid environment, so
I was thinking mostly the latter.

-- Andy Barss

In either case, the answer is probably essentially the same. More
solids on the wood will limit penetration. Limit can be read all the
way from "slow down" (wax) to outright "block" (spar varnish).

This is my opinion in really broad terms based on my experience only,
as this is a testy subject among finishers. Wood finishing is unique
from the stand point that we look for penetration in the finish to
help with adhesion.

Theortetically, longer drying oils will get better penetration and
allow a thicker coat of resin due to increased cure time.
Shorter drying solvent based finishes, specifically lacquer, have much
less resin in them (and with me thinning 20%, even less per coat!)
will dry much faster after the carrier evaporates and leave behind
less resin behind to catalyse.

In a very general way, this lack of penetration coupled with a thin
layer of resin is why you can set a sweating water glass on a lacquer
finished table and leave a ring as opposed to setting a water glass in
a table finished with poly.
After all, the ring under the glass is nothing but water vapor.
Think about it like this: ever spray lacquer on a really humid day
with off temps and get finish blush? Same thing.

Wax provides little or no protection as it gets little or no
penetration. Wax sits on top of the surface and forms a film over the
wood. There are lots of different waxes for different purposes
(liberon wax, butchers wax, finishing wax, buffalo wax and all manner
of colored waxes,) but they all perform in the same manner with some
being better than others. For the most part, the strong point of wax
is to make the surface more abrasion resistant (slicker), and easier
to clean (slicker, less adherance of dirt). Some like that soft feel
that wax leaves, too. That would be the reason I would use it.

Robert