FINISHING SECRETS . . .|
6. Tung Oil
Pure Tung Oil provides a hard and tough surface finish that is absolutely waterproof; impervious to dust, alcohol, acetone, fruit and vegetable acids; and it doesn't darken with age like Linseed and other vegetable oils. All of these benefits come at a price - pure Tung Oil takes forever to dry, it doesn't penetrate the wood surface very well, and it is expensive when compared to other drying oils. Tung Oil is a "reactive" finish, commonly called a "drying" oil, in that it will dry and harden when exposed to air.
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Everyone asks me about Tung Oil, but nobody wants to use it because Tung Oil is not a fast finish. It takes a lot of time. But, it is a simple and forgiving finish, and when done properly, its beauty is unmatched. Sometimes we try too hard to avoid the slow and simple things in our modern high-tech lives.
I have used the stuff for years and I can share what I have learned. Other oils are commonly used in finishes because they are less expensive. Linseed, Soybean, Walnut, Sunflower, Orange, and other fruit, nut and vegetable oils are oils that make suitable finishes for wood. Linseed and Soybean Oil are most often used in commercial finishes. Although neither are a natural "drying oil", the addition of metallic drying agents make them suitable for finishing.
There are enough questions asked about Mineral Oil that it deserves a separate discussion. Mineral Oil is a petroleum product that will stay in a "tacky" liquid state forever. Any finish whose solvents will dilute the Mineral Oil can be applied over a coating of Mineral Oil that has been previously applied to the wood. However, the Mineral Oil combines with the finish and acts as a plasticiser, and the resulting finish will be softer and less durable than it would be without the oil.
Many woodturners use Mineral Oil followed by beeswax for salad bowls, with the result being a softer wax coating than if the wax were used alone. A more durable beeswax finish would be realized if the wax were softened with turpentine for easy application. After the turpentine has evaporated the remaining coating will be pure beeswax, not one that has been softened by Mineral Oil.
A personal opinion that may not be shared by many others is that - If you wouldn't put motor oil on your wood, why would you use Mineral Oil? Mineral Oil is a more highly refined form of motor oil.
Linseed is a common finishing oil and it is available as either "raw" or "boiled". Our discussion would not be complete if it were not included. "Raw" Linseed Oil is just what the name implies, pure linseed oil without any modification or processing. It is a poor finish because it is not a natural drying oil. "Boiled" Linseed Oil has metallic drying agents added to make it suitable for use as a finish. It may have many properties that are similar to Tung Oil, but it turns darker faster than Tung, and it is neither as hard nor as durable as Tung Oil. For these reasons, I don't believe that it is a suitable finish for a piece of turned wood.
There are several modified and partially polymerized forms of Linseed Oil available as a finish. These are sold as Tru-Oil (available as a gunstock finish for many years), Tried and True, Velvit Oil, and probably others as well. Other than drying faster and being slightly more colorfast than Boiled Linseed Oil, I have found no benefits to their being used over any other linseed oil products. A product such as the Minwax Antique Oil Finish will give the same results at a far lower cost.
Modified Tung Oil
Tung Oil is available in pure, modified, and polymerized form. The modified form is what is found in the commercial finishes that contain Tung Oil. Metallic dryers and thinners are used to improve the slow drying rate and poor penetration of pure Tung Oil. Less expensive oils such as linseed, soybean, and other vegetable oils are often substituted for some portion (sometimes all) of the Tung Oil to reduce the cost.
Every effort has been made to make a product that is easy to use, dries quickly, and sells at a competitive price. The problem is that none of the characteristics of pure Tung Oil have been retained. These products have made "Tung Oil Finish" a generic term. Many of them do not contain any Tung Oil. Others may contain a small amount that is hidden in the small amount of varnish that is included as an ingredient. Still others use polyurethane varnish resins and they contain no Tung Oil at all.
The commercial oil and oil/varnish finishes will be discussed in Article 7 of this series.
Pure and Polymerized Tung Oil
The only difference between "pure" and "polymerized" Tung Oil is that the latter has been through a cooking process to partially complete the molecular cross-linking that occurs in a drying-oil. These are often referred to as "partially polymerized" oils.
While pure oil is very slow drying, the polymerized is fairly fast drying because much of the drying time has been used up in the "cooking" process. Both forms are difficult to store. After about a year or two, depending on the humidity, temperature, and exposure to light, they will start to form a film on the surface or a gummy deposit around the edges of the container, and at that time they have to be tossed out. Bloxygen or collapsing storage bottles are recommended for storage.
I know of several sources for the polymerized Tung Oil. Sutherland WellesŇ brand is available from Garrett Wade, Lee Valley Tools, or direct from the manufacturer. It is sold with various quantities of thinners, but the 50% solids ratio is recommended for our use. Add turpentine to thin it and improve penetration for the first (only) coat. After that, use it as it comes from the can. The latest price is about $34 for a quart. Visit their website at http://www.sutherlandwelles.com.
A less expensive source is a product from Woodworkers Supply in their proprietary J.E. Moser® brand. It is called "Polymerized Tung Oil Varnish." The high gloss mix contains 45% solids. It is an excellent product that sells for $17 per quart, half the price of the S-W® brand. Don't worry about the word "varnish" because this term is in common use for anything that develops a surface film.
I used to buy a "Jasco"® brand of Tung Oil, which was polymerized, contained something like 50% solids, and was sold by True Value Hardware. It has since disappeared, but may be available as a special order item from them.
Because of the price and availability, I have also used a lot of 100% pure Tung Oil. It is slower drying, but this will be improved with thinners. I usually use Turpentine, but other thinners can also be used. VM&P Naphtha dries faster than turpentine, and 1-K Kerosene is slower to evaporate. Tung Oil loves to be rubbed, and the more heat generated the faster it dries. This makes it a great "friction polish" for lathe finishing or hand rubbing.
The best pure Tung Oil that I have used is available from Daly's in Seattle at their Stone Way store where they mix all of their products. They will go in the back and draw a quart from the bulk supply that they are using. It is the freshest that I have ever used, and costs about $15 for a quart. I have never been able to talk to their chemist, but I believe that it is partially polymerized as well. At least it behaves that way. Other brands that I have used are "Old Masters"® or "Hope's"®. The Old Masters has served me well over the years and I have always preferred it because the in-store stock has always been fresher.
When applying polymerized Tung Oil, I do a few things differently from the directions on the can. After sanding to 320 dry, I apply a liberal coat of Watco Liquid Finishing Wax, and then wet sand with 400-grit, wipe it off, and allow it to dry. ALL of the surface blemishes will be amplified, and the little surface wax that remains will act as a lubricant when I go back to clean them up.
Then I apply a brushed on coat of Deft Lacquer as a sanding sealer. You may want to use Park's® brand Lacquer Sanding Sealer. I avoid the commercial sealers because they contain opaque zinc-stearate fillers that can mask the grain color and pattern. After sanding to at least 600-grit, I apply a heavy coat of Deft, and immediately wipe it dry with paper towels. Then I leave it to dry for a few minutes, then buff the surface with 0000 steel wool or the gray ScotchBrite, and then repeat the application if there are any rough spots.
Then I apply the oil with a soft cloth. Polymerized is used straight from the can. Put on a thin even coat, and avoid overlaps, if possible. DO NOT wipe it dry as instructed on the can. Let it set 24 hours, or until dry. Truly dry will take three weeks, but it will be cured enough to re-coat when it feels dry and comes up as a white powder when buffed with 0000-steel wool.
Sand after each coat with 0000-steel wool, and apply 4 or 5 coats. Leave the final coat alone. I apply as many coats a required to get the gloss that I desire, and then apply one more.
The only difference when using "pure" is that I thin it to a 4/1 ratio of thinner/oil for the first coat. For subsequent coats, I reduce the ratio of thinner to oil to 2/1 or 1/1, whichever is required for it to flow smoothly in a thin even coat without lines. Other than that, it is applied the same way as the polymerized, except that I don't worry about overlapping, and just keep rubbing until the piece is covered. It does take significantly longer to "dry." I always cure it overnight in my dryer box with the light bulb sized for about an 90°F inside temperature.
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