On Feb 21, 11:16 pm, Andrew Barss wrote:
I always thought the solids percentage was the in-the-can ratio
of solids to solvent (so once the solvent evaporates, the
effective solids go up to 100%, regardless of what they started as).

That is true. However, look at it this way: If you put finish "A" on
that has 20% solids, compared to "B" that is 30% solids, you will have
an extra 10% more solids on the material with just one coat of B
compared to A. With two coats of B, it is like having three coats of
A. Lots more resin on the project.

In the end though, it is about mil thickness. They all dry to 100%
solid resin if they are a curing product.

Does the stated solids amount affect transfer of water vapor? There was
an old FWW article on how well finishes resist that, with thick wax
coming out on top (really thick, not what you'd actually put on
something), followed by shellac, followed by various varnishes,
with oil at the bottom.

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?

Robert