How to Choose a Finish: Part II
For an overview of choosing a finish please refer to “How to Choose a Finish: Part I.” To better understand finishes and their differences, it’s very helpful to put them into categories by the way they cure.
You may think the resins—polyurethane, alkyd, acrylic, etc.—make the big difference in finishes, but they don’t. Consider, for example, that polyurethane resin is used in varnish, water-based finish, two-part finish and in some lacquers. If you have used any two of these finishes, you know they are very different. The curing process the finish goes through is far more significant for understanding each finish and its characteristics.
Let’s review the seven categories of finishes listed in Part I but put them is a slightly different order:
- Oil and blends of oil and varnish (often called “Danish” oil)
- Varnish, which includes polyurethane varnish, spar varnish, wiping varnish (varnish thinned about half with mineral spirits) and gel varnish
- Two-part finishes, which are very protective and durable and which include catalyzed lacquer, conversion varnish, polyester, epoxy resin, UV-cured finish and two-part water-based finishes
- Shellac, which can be clear or amber and with its natural wax included or removed
- Lacquer, which includes the most common, nitrocellulose lacquer, and also colorless CAB-acrylic lacquer and slower drying brushing lacquer
- Wax, which can be used as a finish but is not water resistant and therefore usually limited to objects that aren’t handled much
- Water-based finish, which is often confusingly labeled lacquer, varnish or polyurethane but is an entirely unique finish
Now let’s group the seven by the three methods in which they cure: reactive, evaporative and coalescing.
The first three finishes—oil, varnish and 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 oil and 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.
Oil is unique, however, in that it never cures hard. So oil and blends of oil and varnish can’t be built up to a thicker film. All the excess has to be wiped off after each coat or the finish will dry sticky.
With so thin a film, oil provides no significant water resistance and, of course, it’s too soft to be scratch resistant. So for the sake of meaningful comparisons, it makes sense to leave oil out of this discussion.
The crosslinked network in varnish and two-part finishes makes these finishes 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 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 redissolve after they have cured, however, new coats often don’t bond well to previous coats (after a short time for two-part finishes and several days or weeks for varnish). It’s always a good idea to scuff-sand between coats if a lot of time has elapsed so the coats can interlock and bond mechanically.
Other tradeoffs that come with reactive finishes’ durability have to do with repairing and stripping: Their resistance to heat and solvents makes repairing and stripping more difficult. In addition, their 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.
The next three finishes on the list—shellac, lacquer and wax—dry, or cure, entirely by the evaporation of their solvent. No crosslinking occurs.
Like oil, which was left out of the discussion above, wax doesn’t ever harden and can’t be built up to a protective film. So it makes sense to leave it out of this discussion even though it dries by the same evaporative process as do shellac and lacquer.
Shellac and lacquer are composed of long, stringy molecules that resemble spaghetti on a molecular scale. In a solvent, these molecules float around and become intertwined like spaghetti in a pot of hot water. When the solvent (or water) evaporates, the molecules (spaghetti) form a hard, solid film (mass).
When the solvent (water) is reintroduced, the molecules (spaghetti) first become sticky and then separate and again float around in the solution. This is in contrast to reactive finishes which don’t redissolve in their solvent.
Because of the resemblance to spaghetti, 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 will leave an indentation in shellac and lacquer films. Also, acids in body oils will break down these finishes over time as is often evident on chair arms and backs, and around knobs and pulls on cabinets and furniture.
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 better rubbing qualities. It’s easier to rub shellac and especially lacquer to an even sheen than any other finish.
Because evaporative finishes cure 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, the last on the list, are the only coalescing finish. They cure by both chemical reaction and solvent evaporation. (Latex paint and white and yellow glue also cure 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 microcopic soccer balls 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 very slow evaporating solvent softens them so they stick together.
Then, when the solvent evaporates, a solid film is left just like what happens in an evaporative finish.
Coalescing finishes are thus very protective and durable at the location of the droplets, but penetrable and dissolvable at the points where the droplets join. Water vapor passes through these areas 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 an overwhelmingly large percentage of the surface area, coalescing finishes are highly scratch resistant—enough so that they can be used effectively on floors.
Coalescing finishes are also relatively difficult to recoat, repair and strip once curing has occurred, because most of the film is reactively cured. As with the 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, curing time depends almost entirely upon weather conditions—very slow when it’s cool or humid, very fast when it’s hot or dry. As a result, you may have some issues applying water-based finishes in intemperate weather conditions.
Classifying by their curing properties is the best way to make sense of finishes.
You may laugh at first at my using Tinker Toys, spaghetti and soccer balls to describe these types of finishes, but I find the analogies very helpful. I’m often asked to explain why a finish is doing something or other, and I find myself picturing one of these objects in my mind to come up with the solution.
TABLE: COMPARING THE THREE CATEGORIES OF FINISH
The Finishes Moisture Resistance Scratch Resistance Solvent, Heat, Acid and Alkali Resistance Ease of Repair and Stripping Ease of Rubbing to an Even Sheen Ease of Dust-Free Application Reactive Finishes (except oil)
Excellent Excellent Excellent Good (varnish); Poor (two-part finishes) Poor Poor (varnish); Excellent (two-part finishes) Evaporative Finishes (except wax)
Good Good Poor Excellent Excellent Excellent Coalescing Finishes
Water Base Good Very Good Poor to Good Good Good Very Good