All Wood Finishes Go Into Just a Few Simple Categories

The Choices are Overwhelming, but We Can Simplify It!

Go into any woodworking store and you will see a staggering array of different wood finishes. We need to simplify wood finishes! How to choose?  Asking the community at one of online woodworking forums is of little help.  Almost everyone who responds to your question will have a different answer.  Part of the problem is that the manufacturers make things opaque, confusing, and intentionally misleading. Fortunately, we can put all of them into just a few simple categories.

Two Simple Ways to Categorize Wood Finishes

You could probably categorize finishes in many different ways, but just two broad categories will go a long way towards simplifying things.  The first is to categorize them by how they dry, or cure.  The second is by whether they build a film on the surface. This first chart (Flow Chart – A) shows how almost every wood finish available on the market can be put into just three categories.  One type of finish that is not shown on this chart is non-drying oils like Mineral Oil.  I did not include these because they do not cure and they do not dry.  They just soak into the wood and stay there.

The largest category that encompasses the most products is the Reactive category.  These finishes cure with some kind of catalyst.  The most common catalyst is oxygen.  Traditional varnishes, polyurethane, and drying oils like linseed and tung oil all cure by exposure to air.

What this means is that once they are exposed and are fully cured by the catalyst that they are chemically and physically changed.  The example that I use to show this is with polyurethane.  Before it is fully cured by exposure to oxygen, polyurethane is soluble in mineral spirits.  If you spill polyurethane or a similar product, then you can clean it up and remove it with mineral spirits because it is “soluble” in mineral spirits.  But, once polyurethane is fully cured then it is impervious to mineral spirits.  You can wash and clean polyurethane with mineral spirits once it is fully cured and the same product that would dissolve it beforehand will have no effect whatsoever. That is what I mean when I say that these products are changed by the curing process.

Two part finishes like 2k lacquers and epoxy also cure primarily by the addition of a catalyst, but it is a man made chemical catalyst rather than air. These might be affected by air, but they fully cure with a catalyst.

UV cured finishes are also reactive and cure by a catalyst, but in this case the catalyst is sunlight or UV lamps. You can a leave UV cured finish in an open containers for days without affecting it provided that it isn’t exposed to UV light.

Evaporative finishes  There are only two finishes in the evaporative category that I know of, but they are very important to the field of wood finishing.  They are shellac and non-catalyzed lacquer. 

Evaporative are soluble in certain solvents. For shellac, that solvent is generally alcohol and for lacquer it is generally lacquer thinner. 

It’s easiest to explain how evaporative finishes work by explaining how shellac works.  Shellac is often sold in solid form as flakes. These flakes are soaked in alcohol.  The alcohol dissolves the flakes to create a liquid form of shellac that we can apply to our projects. The shellac returns to a solid form as soon as the alcohol evaporates.  What we are left with is a very thin layer of  solid shellac. 

The difference between this and the reactive finishes is that the shellac did not cure and it was not chemically changed.  It simply dissolved in the alcohol, was distributed onto our project, and returned to a solid form once the alcohol evaporated.

The biggest advantage to this is that the shellac is still soluble in alcohol.  And, that makes it very easy to repair.  If the finish is damaged then we can just apply some more liquid shellac over the damaged area and the new layer of shellac will melt into.