Super Smooth Oil Finish
By Sal Marino
Many woodworkers like to use oil finishes instead of lacquer, varnish, polyurethane or waterbased finishes because oil finishes are much easier to apply and much more forgiving when mistakes happen. However, one of the main drawbacks of an oil finish is that one cannot achieve a glass smooth finish on open pored woods (like oak, mahogany etc.) like one can when applying a lacquer, varnish poly or other topcoat finish. When using a topcoat finish on open pored woods, you can either build up the finish by applying multiple coats and sanding back down until the pores have been filled or you can first apply a paste filler to fill the pores, then apply a topcoat finish. Oil finishes are thinner and contain much less solids than topcoat finishes, therefore it would not be practical to apply multiple coats and sand back until the pores have been filled. This would take much too long. Also oil finishes need to penetrate the wood in order to work properly. Once the finish penetrates, the solvents evaporate and the resins solidify actually making the wood itself harder. If the pores have been filled with paste filler (which is silica) a very finely ground glass, the oil finish will not be able to penetrate the filler.
While leaving the pores open when using an oil finish is OK and many times even desirable on some pieces of woodwork like a chest of drawers, chair or clock, for other pieces like conference tables, dining tables, pianos, jewelry and music boxes may look much better if the pores were filled so a glass smooth finish can be obtained. In the end, it still comes down to solely a matter of taste.
Many years ago, I read somewhere that one could wet sand some oil finishes to achieve a higher sheen. The article mentioned nothing about whether this was to be done with open or closed pored woods. At the time I was using an oil finish that I still use quite often today, it is called Watco Danish Oil. This finish is very easy to apply and leaves a beautiful satin to semi-gloss sheen, depending on how many coats are applied. It should only be applied to raw wood so it can penetrate properly. It is available in a natural as well as many colors such as shades of walnut, cherry, golden oak and others. I decided to run some tests using the Watco natural color on various hardwoods. I first tried cherry and maple. First I prepared the wood in my normal manner by sanding with coarse, medium and then fine paper. I then applied a generous amount of Watco Danish Oil to the surface and started to wet sand with 600 grit silicon carbide wet or dry paper. After applying 3 coats of the oil (one per day) and wet sanding each coat, I compared the wet sanded pieces with samples of maple and cherry that I just applied three coats of oil (without wet sanding). The results were disappointing. I did not see any signs of the wet sanded samples having any higher sheen than the pieces that were just oiled. If anything these pieces were a little duller than the samples that were not wet sanded. Next I ran some tests using oak and walnut. I proceeded in the same manner as described above, but this time the results were much different. The wet sanded samples did have somewhat of a higher sheen, but what was more impressive to me was that the surface of the wet sanded pieces of oak and walnut were much smoother than the samples of oak and walnut that were just oiled with no wet sanding. It was then that I realized what had happened, why the surface was smoother and why the sheen had increased. By wet sanding, the Watco Danish Oil mixed with the sawdust that was being created by the sanding. This created a sort of slurry or paste. As I continued to sand, the paste was forced down into the pores of the wood. Basically I had filled the open pores of the oak and walnut by using its own sawdust in combination with the oil which worked as not only a finish but also a binder to hold the sawdust down in the pores and level the surface. The reason why a higher sheen was achieved was simple. Once the pores are filled, much more light reflects off the surface in contrast to when the pores are open the light gets trapped in all the nooks and crannies of the open pores. The author of that article I read must have been using open pored wood.
Although the sheen was somewhat higher by wet sanding on open pored wood, there was not a dramatic difference. I believe the author of that article missed the most important advantage of wet sanding. That is, of course, being able to fill the pores of wood to achieve a glass smooth surface when using an oil finish. Now the term glass smooth may be somewhat confusing. many people associate this with producing a high gloss finish. This is not true. You will not get a high gloss sheen (like you would when using lacquer, varnish or other topcoat) when using any oil finish. The term glass smooth refers to how level the surface is and how smooth it feels.
Over the years I have developed and refined the process of wet sanding on open pored
wood. The following is the method I currently use:
Watco Danish Oil can be purchased at special reduced price from Constantine's