Most people try making sense of finishes by identifying the resin they’re made from: polyurethane, nitrocellulose, alkyd, acrylic, etc. But doing this doesn’t work well.
Consider, for example, that polyurethane resin is used in varnish, water-based finish, two-part finishes and in some lacquers. If you have used any two of these finishes, you know they are very different. The curing or drying process the finish goes through is far more significant for understanding each finish and its characteristics.
There are three large categories of finishes, grouped by the way they cure or dry: reactive, evaporative and coalescing. Understanding these three categories is equivalent to understanding the tools used in woodworking. Just as the tools are the first thing taught in a beginning woodworking class, the three categories of finishes should be the first thing taught when learning finishing.
Varnish (including polyurethane varnish) and all two-part finishes cure by a chemical reaction that takes place after the thinner has evaporated. The reaction is brought about either by the absorption of oxygen in the case of varnish, or by the addition of a second part (often called a catalyst, hardener or crosslinker) in the case of two-part finishes.
Because the chemical reaction causes the molecules in these finishes to join up or “crosslink,” you can picture reactive finishes as Tinker Toys on a molecular scale. It’s the crosslinked network that gives reactive finishes their defining characteristics. It makes them very protective and durable.
They are protective because there is so little space between the molecules in the tightly networked cured film for water or water vapor (humidity) to easily get through.
They are durable because objects have to be quite coarse and considerable pressure has to be applied to tear apart the chemical bonds enough to make a scratch. And it takes high heat (from a heat gun, for example) or strong solvents, acids or alkalis to soften and stretch the bonds enough to cause the film to blister or separate from the wood.
Because reactive finishes don’t re-dissolve after they have cured, new coats often don’t bond well to previous coats. It’s a good idea to scuff-sand between coats if a lot of time has elapsed so the coats can bond mechanically.
Other tradeoffs include invisible repairs and stripping. These are more difficult because of the resistance to heat and solvents. In addition, the resistance to scratching makes rubbing to an even sheen more difficult to achieve.
Also, in the case of varnishes, because it takes a long time for oxygen to penetrate and bring about the crosslinking, these finishes take significantly longer than others to become dust-free.
Lacquer and shellac dry entirely by the evaporation of their solvent. No crosslinking occurs. These finishes are composed of long, stringy molecules that resemble spaghetti on a molecular scale. In a solvent, these molecules float around and become intertwined like stirred spaghetti in a pot of hot water. When the solvent evaporates, the interlocked molecules form a hard, solid film. And when the solvent is reintroduced, the film first becomes sticky and then re-dissolves.
Because of the resemblance, I find it helpful to think of evaporative finishes as “spaghetti” finishes in contrast to the reactive “Tinker-Toy” finishes.
Just as with spaghetti dried and hardened in a pot, evaporative finishes always have microscopic spaces through which tiny water molecules can pass. As a result, evaporative finishes are not as moisture-resistant as reactive finishes.
Nor are evaporative finishes as durable as reactive finishes. The forces that hold the stringy molecules together are very weak and the molecules separate easily when scraped by coarse objects or brought in contact with heat, acids, alkalis or many solvents.
For example, no more heat than that from a hot coffee cup can leave an indentation in shellac and lacquer films. Also, acids in body oils break down these finishes over time as is often evident on chair arms and backs and around knobs and pulls on cabinets.
On the other hand, weak resistance to solvents makes evaporative finishes the easiest of all finishes to recoat, repair invisibly and strip, and their susceptibility to scratching translates into easier rubbing to an even sheen.
Because evaporative finishes dry entirely by solvent evaporation, drying time varies according to the solvent’s evaporation rate. This is usually quite rapid, so evaporative finishes present few dust problems.
The long, stringy nature of the molecules, however, requires a lot of solvent to separate them enough so they can be sprayed through the tiny orifice in a spray gun without getting severe orange peel. This is becoming an increasing problem in some areas because of environmental laws limiting the use of solvents.
Common water-based finishes are the only coalescing finish. They dry by both chemical reaction and solvent evaporation. (Latex paint and white and yellow glue also dry in this combined way.)
A coalescing finish is made up of tiny droplets of reactively cured resin dispersed in solvent and water. Within the droplets, which you can think of as microscopic soccer balls or peas with a Tinker-Toy-like network inside, the resin is crosslinked. As the water evaporates, the droplets “coalesce” (that is, they come together) and the slow evaporating solvent softens them so they stick together.
Then, when the solvent evaporates, a solid film is left just as in evaporative finishes.
Coalescing finishes are thus protective and durable at the location of the droplets, but penetrable and dissolvable at the points where the droplets join. Water vapor passes through at about the same rate as through shellac and lacquer (think of the “breathing” characteristic of latex paint).
Also, heat, many common solvents and relatively weak acids and alkalis can break down the film at these points.
But because the droplets are reactively cured and make up almost the entire surface area, coalescing finishes are scratch-resistant — enough so that they can be used effectively on floors.
Coalescing finishes are also relatively difficult to recoat, repair invisibly and strip when dry, because most of the film is reactively cured. As with purely reactive finishes, you should sand between coats to create a mechanical bond unless you apply the coats within a few days of each other.
Because the evaporation rate of water is tied so closely to temperature and humidity, drying time is influenced more by weather conditions — very slow when it’s cool or humid, fast when it’s hot or dry. As a result, you will have more problems applying water-based finishes in intemperate weather conditions.
Bob Flexner is the author of “Understanding Wood Finishing” and “Flexner on Finishing.”
This article originally appeared in the November 2016 issue.