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Super Smooth Oil Finish
by Sal Marino

Watco Danish OilMany woodworkers like to use oil finishes instead of lacquer,  polyurethane or water based 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.) when using oil finishes. 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 in many cases contains 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, other pieces like conference tables and pianos 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 wood tone colors such as walnut, cherry, golden oak and others.

I decided to run some tests using the WatcoŽ 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 WatcoŽ 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 WatcoŽ (without wet sanding). The results were disappointing. The wet sanded samples did not have 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 WatcoŽ 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 left open the light gets trapped in all the nooks and crannies of the open pores and therefore a lower sheen results. 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 "Glass Smooth" with  a high gloss finish. This is not true. When using any oil finish, you will not get a high gloss sheen as you would when using lacquer or other topcoats. The term glass smooth refers to how level the surface is and how smooth it feels.

The Materials:
Watco Danish Oil
320 Grit wet or dry sandpaper
Sanding block
Pastewax

The Process
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:

1. Prepare the wood by sanding with coarse (100 grit), medium (180 grit), and fine (240 grit) sandpaper. Make sure to wipe off all sawdust after sanding.

2. Apply a generous amount (almost flood the surface) of WatcoŽ Danish Oil to the surface.

3. Over the years I have found that it is better to use 320 grit silicon carbide wet or dry paper rather than 600 grit. The 320 grit paper will create the paste quickly thus fill the pores better. Immediately after the WatcoŽ Danish Oil has been applied (while it is still wet on the surface) wrap a piece of 320 paper around a sanding block and start to wet sand with the grain. Continue to oil and wet sand until you feel enough of the paste has been worked down into the pores.

4. There will still be a substantial amount of paste left on the surface. Don't wipe it off right away. Let the surface dry for about 10 minutes, then wipe off the excess paste using a lint free rag. Wipe against the grain, trying to cut the paste off at the surface, this way the paste in the pores will remain and not be pulled out. Let dry overnight.

5. Before you continue, there may be a small amount of paste that is still on the surface. This needs to be removed, if not visible now, it will be be when you apply additional coats. Because this has dried overnight, you will need to sand it off. Take another piece of 320 grit paper, wrap it around a block and DRY SAND lightly with the grain. You need only to take a few passes, just enough to remove any excess paste that has remained on the surface. Inspect the work piece by shining a drop light down at approx a 30 degree angle to the surface.  Any remaining paste will show up easily.

6. I have also determined that in most cases wet sanding need only be done on the first application. Therefore, you need only wet sand once (in step 4). After, it is just a matter of applying additional coats of WatcoŽ (without wet sanding) until you achieve enough protection and the desired sheen. Usually I apply three to four additional coats after the first wet sanding coat. I let each coat dry overnight and very lightly scuff between coats with 0000 steel wool.

7. After the last coat has been applied, I let the finish cure about 1 week and apply a coat of quality paste wax.

That's it. Try it, I am sure you will be very happy with the results.

 
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