What is That Finish?
There is a series of simple tests that can be used to quickly identify (or at least eliminate from consideration) just about any finish you are likely to encounter. Before you conduct these tests, however, the item in question should be thoroughly cleaned. A build-up of dirt, wax, grime, etc. on the surface may very well give you a false indicator with any of the following test. So, before making any of these tests, give the piece (or at least the area to be tested) a good cleaning. I begin with a mild detergent and warm water which I follow with a thorough wipe down with mineral spirits after the water is dry. When you wipe with mineral spirits I suggest that you use paper shop towels and turn the towel to a clean face with every wipe. In this way you remove the contaminants rather than simply softening them and spreading them around.
To conduct these tests you will need denatured alcohol, xylene, and lacquer thinner. I keep a simple “test kit” in which I include these solvents in small squeeze bottles. In addition, the kit has several paper shop towels and a small squirt bottle of mineral spirits.
Begin by applying a few drops of alcohol. If the finish is shellac (or “spirit varnish”) in short order the alcohol will begin to dissolve the finish and it will becomes noticeably soft and a bit sticky to the touch. If nothing happens, move to step two.
Next, apply a few drops of lacquer thinner to another area. If this causes the finish to soften then it may be lacquer. The next question is; how old is the piece? If it is over ten to fifteen year old (and has not been refinished recently) it is most certainly lacquer. However, if the piece was made (or refinished) within the last fifteen years it may also be a water-borne finish—lacquer thinner will also soften water-borne finishes. To test for water-borne proceed to step three.
Finally, apply a few drops of xylene in another area. If the xylene softens the finish it is most definitely a water-borne finish.
If none of the preceding steps softened the finish, you are looking at one of the “reactive finishes”—finishes that cure by a chemical reaction and are no longer dissolved or softened by solvent. For example, mineral spirits will do nothing to soften varnish once the varnish is cured. If this is the case you have a varnish finish (polyurethane, alkyd, or phenolic resin) or you have one of the two-part finishes (conversion varnish or catalyzed lacquer) that are becoming more common in commercial furniture and cabinetry.
Important Note: The order in which you perform these tests is very important. For example, lacquer thinner will also soften shellac. Therefore, if you reverse the order of the first two steps by applying lacquer thinner first, and it softened the finish, the result will have told you very little. Lacquer thinner will soften shellac, lacquer, and water-borne finishes.
The cleaning step is also very important. A build up of wax, furniture polish, or simple household dirt and grime will be softened by any of these solvents. Therefore, if the piece is "dirty" when you perform the tests then all of them will produce a "positive" result.