I think enough of oil finishes, and Tung Oil in particular, to give it a separate article for discussion and comparison. I have used the stuff for years and I can share what I have learned.
Everyone asks me about Tung Oil, but nobody wants to use it because Tung Oil has a reputation for not being a “fast” finish. It does take 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.
Why Tung Oil
Why am I such an advocate for Tung Oil? Most finishing texts say there is no difference between what oils are used. If that were true, we could get the same results from the least expensive oil.
It may be true that there is little difference in the short term or when used on a table top that will spend most of its life covered with a cloth. However, I believe that whoever receives something that I have turned and finished is entitled to expect that the finish will last for many years, without deterioration or change in color when placed in the exposure and handling of a normal household environment. It is my experience that Tung Oil is the only oil that meets these criteria.
Tung Oil provides a 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.
Other Oils
There are other oils commonly used in finishes. Linseed, Soybean, Walnut, Sunflower, Orange, and other fruit, nut and vegetable oils can make suitable finishes for wood. Many people will use Mineral Oil, either by itself or with other ingredients added, as a wood finish. I will discuss those that I have used as separate topics.
What is a "Drying Oil"?
There are two (2) types of oil finishes, those that dry and those that don't. The "drying" oils don't really "dry". They only appear to "dry" as they harden when they are exposed to air. What really happens is a process called "polymerization", where the smaller molecules of the liquid cross-link to form the larger molecules of a solid. This reaction uses the oxygen from the air.
The so-called "Non-drying" oils will generally not "dry" or the process is so slow that they remain tacky forever. However, some of the "non-drying oils" can be forced to cure through the addition of an oxidizer in the form of a "drying agent". Linseed and Soybean Oil are most often used in commercial finishes. Neither are truely a natural “drying oil”. Raw Linseed Oil will cure on the surface, effectively sealing the liquid oil under it from the oxygen supply in the air, with the result that it will take forever to cure into a hard finish on the wood, if at all. Soybean Oil in its natural state will not cure at all. The addition of metallic drying agents make both Linseed and Soybean oils suitable for finishing. We recognize the modified Linseed as "Boiled Linseed Oil". And, we find Soybean oil (with drying agents) used in many commercial finishing products because it is readilly available at a lower cost.
Any edible non-drying oil can be used as a coating on wood. Non-drying oils that are useful coatings for a salad bowl would be Mineral Oil, Sunflower Oil, any of the vegetable cooking oils, and pressed Walnut Oil. Choose the oil carefully, clean and renew often, because many of the vegetable oils and most cooking oils will turn rancid with time.
My preference is the SUNflower Oil (not to be confused with Safflower Oil) because it does not turn rancid with age, it has a pleasant odor, and it paretially dries.
Modified Tung Oil
Tung Oil is available in several forms, modified, partially polymerized, and pure. These are discussed separately.
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 and soybean oils are often substituted for some portion 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 very few, if any, of the characteristics of pure Tung Oil have been retained. These products have made “Tung Oil Finish” a generic term. Some of them do not contain any Tung Oil at all, although they use the name "Tung Oil" on the label, even after it has been 100% replaced by less expensive oils. Others may contain a small amount that is hidden in the small amount of varnish resins that are 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 partially polymerized oil 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 Welles brand is available from Garrett Wade, Lee Valley Tools (with their house label), 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. Some think it is worth the cost. I don't.
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.
Because of the price and availability, I have also used a lot of 100% pure Tung Oil. It is slower drying, but this can 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. Pure Tung Oil is available for about $15 per quart.
Someone always comes up with the argument that there is no such thing as “100% pure” Tung Oil because it has all been processed to some degree, and that thinners and preservatives have been added. My response is that, if it says “100% pure” on the can, it is close enough for me.
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.
Another excellent Pure Tung Oil is available from Swing Paints. Other brands that I have used are “Old Masters” or “Hope’s”. The OM has served me well over the years and I have always preferred it because the in-store stock has always been fresher.
Other Oils
Although I prefer Tung Oil for a finish on turned wood, the other oils have their place in our collection of finishing oils.
Mineral Oil
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. It is definitely a non-drying oil. There is no difference between Mineral Oil and the motor oil we use in our engines, except that Mineral Oil is more refined, contains no additives, and has been certified by the FDA for human consumption. 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?
Any finish whose solvents are compatible with the Mineral Oil can be applied over a coating of Mineral Oil that has been previously applied to the wood. This really means that an oil or varnish can be applied over a Mineral Oil because they use the same petroleum based thinners that can also be used to thin the Mineral Oil. The thinners used in shellac, lacquer, and waterborne finishes are not compatible, and these finishes should not be used over Mineral Oil. Although it appears to be compatible, I would not use a Turpentine thinner with Mineral Oil because the essential oils in the Turpentine are not compatible and they will separate in time.
When a finish is used over Mineral Oil, the mineral oil combines with the finish and acts as a plasticiser. The resulting finish will be softer and less durable than it would be without the oil. How soft depends on how much Mineral Oil has been absorbed into the wood.
Many Woodturners use Mineral Oil with beeswax for salad bowls. The result is 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.
Linseed Oil
Linseed Oil is such a common finishing oil that our discussion would not be complete if it were not included. Linseed is available as either “raw” or “boiled”. “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.
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 similar results at a lower cost.
