Bill Tindall, E.Tn.
If memory serves, for a long time finishes generally have been classified as being either penetrating or film forming, with no suggestion of a middle ground.....baloney, they all penetrate
But it is never explained how the size of molecules in the various finishes compares to the diameter of the spaces between and within wood cells – can the molecules of finish actually fit within these spaces? If they are small enough, do they actually enter these spaces? ...I'll explain it. The pores in wood are a bizillion times bigger than a finish molecule. The spaces in the solid part of wood is even bigger that a finish molecule before curing. You know that . Water molecules come and go freely through wood.
BUT, it is not the size of the molecule that determines whether the finish penetrates the pores. Penetration is determined by the surface tension of the wood vs finish. However, if the finish wets the wood, it penetrates.
The three most common solvents used to thin finishes may be mineral spirits, denatured alcohol, and water. All three are rapidly absorbed into raw wood. So when a finish containing one of these solvents is applied and the surface “becomes dry,” are molecules of finish being drawn into the spaces in the wood with the solvent......yes
Does this mean shellac can be both a film forming and a penetrating finish, and if so what is the value of the classification? ....none
Can the same questions be asked of varnish straight from the can versus the same varnish thinned 50% with its solvent? same answer
would seem to offer a different result than one or the other....nope
Does a sprayed lacquer sanding sealer penetrate the wood the same as a brushed one pound cut of shellac? Both are sometimes simply called “sealers” but do they interact with raw wood identically? yes for any practical purpose
Is this film conceptually different from the film resulting from straight-from-the-can varnish? no
This whole discussion hinged on the flawed concept of "penetrating and film forming finishes." They are all the same. Only the amount of solids differs. It takes more solids to form a film that builds on the surface.
Well there is another effect. Oil will penetrate farther in wood because its viscosity remains low for a long time. The equivalent varnish will begin to cure and become so viscous it not longer moves into the wood. Cured varnish essentially becomes one molecule and that indeed is too big for the pores.