When you want to apply a clear finish to a wood table or cabinet, you have a choice between penetrating oils and varnishes. Tung oil, which some people misspell as "tongue oil," is an example of the first option, while urethane – or more commonly polyurethane – is an example of the second. Oils sink deep into the wood, and although they harden in the grain, they never prevent the addition of more oil. That's not true for varnishes. Finishers refer to them as film finishes because they create an impervious coating that won't permit oils to pass. Consequently, if you like the idea of applying tung oil finish, don't apply varnish first. The tung oil will just sit on top of the varnish and leave a sticky deposit.
Oils Come Directly From Nature
Historically, wood crafters have been using oil finishes longer than they've been using varnishes. The two most common oils are linseed oil, derived from flax seeds, and tung oil, which comes from the nut of a tree that grows in China. Tung oil is a fairly new addition to the North American woodworking scene, having been introduced in the early 1900s. Now that it's here, it's a common ingredient in many finishing products that aren't actually oils, despite what the labels on these products say.
What Is Varnish?
Most varnishes contain tung or linseed oil as an ingredient, but they also contain natural or synthetic polymers. The most common polymer was a plant-based alkyd until paint manufacturers discovered how to dissolve plastics, such as acrylic and polyurethane, in a mixture of solvents and natural oils. These polymers harden upon contact with the atmosphere to create a moisture-proof film.
Even though varnish might contain tung oil, the two are as different as bread is from yeast, which is only one of the ingredients in bread. Once varnish hardens and cures, it's impenetrable. When shopping for finishes, you might come across tung oil polyurethane. This product contains tung oil, but because it also contains polyurethane, it's a varnish. You can't use it and pure tung oil on the same piece of furniture.
Applying Tung Oil Finish
If you want to get the best results when using tung oil, you have to apply several coats and scuff each coat with fine sandpaper when it dries. Each coat penetrates into the wood grain and dries in about 4 to 8 hours, and successive coats add more material to the grain and build a deep and natural-looking luster. Boiled linseed oil also dries in the grain, but it does so more slowly, and it doesn't harden as much. Consequently, you usually don't scuff between coats, and for this reason, the ultimate finish isn't as lustrous as that of tung oil.
Neither tung oil nor linseed oil are film coatings, so the wood grain never loses its natural texture. When you apply varnish, however, the product leaves a hard coating on the surface of the wood, and you can build this up to a glassy smooth finish by adding successive coats. Forget about applying pure tung oil to a surface with a varnish coating. The oil can't penetrate, and with nowhere to go, it simply sits on the surface and turns into a semi-hard gel.
Danish Oil vs. Tung Oil
Danish oil and teak oil aren't really oils. They're blends of oil and varnish known as wiping varnishes. They usually contain tung or linseed oil, but they also contain polymers that harden to leave a film coating, albeit these polymers are highly diluted in the oil and solvent substrate. Because wiping varnishes are so thin, you usually apply them with a rag, wiping them on as you would a true oil. They penetrate to a degree, but they also leave a film coating, so you shouldn't use pure tung oil on a piece of woodwork to which you've already applied Danish or teak oil.