Though you may use only one or two finishes in your work, you’ve surely wondered about the others and how they compare. You’ve probably even asked yourself if you shouldn’t be using one of them instead.
To help answer this question, you may have tried to classify finishes by their resins – polyurethane, alkyd, acrylic, etc., but then realized that this isn’t very helpful. Take polyurethane, for example. It is used in oil-base varnishes, water-base finishes, some lacquers and some two-part finishes. If you’ve used any two of these finishes, you know that they are very different.
A much better way to make sense of finishes, so you can choose intelligently among them, is to combine them into three groups by the way they cure, then associate each of the groups with familiar objects – Tinker Toys, spaghetti and soccer balls. This may seem silly at first, but the objects make the groups easy to remember and the groups allow you to figure out the answers to most of your questions, even though you may never have used the finishes.
The three groups are reactive, evaporative and coalescing.
Oil, varnish, and two-part finishes are reactive finishes because they cure by a chemical reaction that occurs in the finish when it comes in contact with oxygen (oil and varnish) or has a catalyst added (two-part finishes). Since the chemical reaction causes the molecules in the finish to join up or “crosslink,” you can picture reactive finishes as Tinker Toys on a molecular scale that link up in a very large network. (See photo #1)
Shellac, lacquer and wax cure entirely by the evaporation of their solvents (there is no chemical reaction and no linking up), so they are evaporative finishes. These finishes are made up of relatively large molecules that are long and stringy in shape, making the finishes resemble entangled, molecular spaghetti. (See photo #2)
If you let all the water evaporate out of a pot of actual spaghetti, it hardens. If you then reintroduce water, the spaghetti first softens and becomes sticky, then the individual strands separate. The same happens to shellac with alcohol, lacquer with lacquer thinner and wax with turpentine or a petroleum distillate, only on a molecular scale. Water-base finishes cure both by chemical reaction and liquid evaporation. Like latex paint and white and yellow glue, water-base finishes are composed of tiny droplets of finish suspended (“emulsified”) in water and solvent. These droplets are very large compared to the molecules in the other finishes. Inside each droplet the finish molecules cure by chemical reaction, but the droplets themselves join only as a result of the water, and then the solvent, evaporating. (See drawing #3)
You can picture the droplets of cured finish as microscopic soccer balls. As the water evaporates, the soccer-ball-like droplets approach each other, or “coalesce,” so water-base finishes are classified as coalescing finishes. The small amount of organic solvent in water base then softens the outer surface of the droplets so they stick together when they come in contact. The solvent then evaporates and a film is formed.
Putting water back onto a cured water-base finish doesn’t cause any damage, but a strong organic solvent like alcohol or lacquer thinner will make the finish sticky, and dull it or dissolve it, just like the solvent does to evaporative finishes.