All of the faithful readers of this column (Hi, Mom!) have said I must know something other than how to make a mess and clean it up. Although I have far more experience with making a mess (and occasionally cleaning it up), here’s some info about wood finishes. We’re going to be stealing a few ideas from Bob Flexner’s definitive Understanding Wood Finishing.
As you know, the purpose of a finish is to protect the wood underneath. Describing them in terms of how they work, there are 3 types: Evaporative, Reactive and Coalescing.
Evaporative finishes, which include shellac and lacquer, are long stringy molecules (think spaghetti) suspended in an evaporative liquid. The liquid is usually something that will evaporate quickly like alcohol. Shellac is a resin secreted by the lac bug and approximately 1.5 million (“lac” means “one hundred thousand”) bugs are required to make a pound of shellac. The resin is scraped from the branches, melted, the bits of bugs strained out, and what’s left is dried and flaked. It’s then dissolved in alcohol to use. Lacquer, a synthetic product that became popular during the 1920’s, caught on because it is more resistant to heat, water, alcohol and acids. Most lacquers are made from nitro-cellulose which comes from treating the fibers of wood or cotton with nitric and sulphuric acids. An important factor in all evaporative finishes is how fast the solvent evaporates. When you put on multiple coats of an evaporative finish, the solvent redisolves the bottom coat and the two mingle and attach to each other, like dumping a pot of fresh spaghetti on top of some dried spaghetti.
Reactive finishes are varnish and all have 2 parts. As the thinner that separates the two molecules evaporates, the 2 molecules touch each other and with either oxygen (as in a varnish) or with a special hardener a cross-linking of the two molecules takes place. The reason this is important is because when you add multiple coats of a reactive finish, you need to do so within hours or days of each other so the top coat can react chemically with the bottom coat to join the two together. Otherwise you need to scuff the surface with a fine sandpaper so the finish can attach mechanically.
A water-based finish is a coalescing finish. The little droplets of finish have solid insides of a reactive finish that has already bonded and an outside coating of a solvent. As the water evaporates, the solvent bonds the little droplets together. White and yellow glue works the same way. The reason you can add water on top of a cured polyurethane finish is because it is really the solvent that is holding the bits of finish together. You can add another coat of a water based finish anytime after the first, because the solvent will help bond the new coat to the old one.
So, you might ask, what is that stuff that’s on my furniture? Alas, the only way to tell is to try and dissolve the finish. Add a few drops of alcohol in an inconspicuous spot. If the finish gets soft and sticky within seconds, it’s shellac. Now apply a few drops of lacquer thinner. If it gets soft and sticky it’s shellac, lacquer or water based. Fortunately, you’ve already ruled out shellac. To tell the difference between water based and lacquer, add a few drops of toluene or xylene. If it gets gummy, it’s water based. If nothing happens, it’s a reactive finish, you just don’t know which one.
Or, you can always guess. Water based finishes didn’t really become popular until the 1990’s so if it’s a recent piece of furniture, that’s probably what it is. Water based finishes have the advantage of less solvents and hence less nasty chemicals going into the air…a good thing for everybody.