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.
Raw wood is not a solid substance like plastic or metal – there are air spaces between the cell walls and within them. The larger cells in red oak for example are visibly hollow to the naked eye. 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?
Sometimes the rationale that a finish is penetrating is the statement that when a finish is applied to wood the surface quickly becomes dry,“ implying the finish molecules penetrate into the wood.”
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, or just the solvent?
A three pound cut of shellac is relatively thick, and many might label it a film forming finish. A one pound cut of the same shellac is much less viscous, and often is suggested as a “sealer” to reduce or eliminate stain blotching on cherry after light sanding, as it does not completely block absorption of stain into the wood. The implication would seem to be that this “wash coat” or one pound cut shellac leaves some molecules of shellac within the spaces of the wood and some molecules on the surface. Does this mean shellac can be both a film forming and a penetrating finish, and if so what is the value of the classification?
Can the same questions be asked of varnish straight from the can versus the same varnish thinned 50% with its solvent?
If a thinned version of a finish “penetrates” the wood while a thicker version forms a film on top, what would be the benefit of applying several coats of the thinned finish to fill the spaces between and within the wood cells, and then brushing a thicker version of the same finish to create a film on top of the filled wood? I’ve never seen this suggested or recommended, but at first glance it would seem to offer a different result than one or the other.
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?
If memory serves, boiled linseed oil and pure tung oil are usually or always classified as penetrating finishes. But if enough coats are applied, allowing adequate time for each coat to cure, boiled linseed oil and pure tung oil eventually build a layer on top of the wood. Is this film conceptually different from the film resulting from straight-from-the-can varnish?
With time, woodworkers come to know what results to expect from finishes they use repeatedly, perhaps without an accurate understanding of the answers to the above questions. But for less experienced woodworkers, marketing statements and casual conversation can be very confusing and even misleading. I might be wrong, but it would seem to me that well defined and carefully used terms and definitions, and understanding of what is actually happening when finish meets wood, would be tremendously beneficial to all.