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Tung Oil

Tung Oil as a Woodworking Finish

What it is

Tung oil, also known as China Wood Oil, Lumbang oil, Noix d'abrasin (fr.) or simply wood oil, is made from the seed kernels of the Tung tree (Aleurites fordii and Aleurites montana, family Euphorbiaceae). The A. fordii tree grows well in cooler climates, but can survive up to sub-tropical climates. A. montana is restricted to a more tropical climate. China, Argentina, Paraguay, Brazil and the USA are all major producers of tung oil.

Tung oil has been known about for hundreds of years in China, where it was used as a preservative for wood ships. The oil penetrates the wood, then hardens to form an impermeable hydrophobic layer (repels water) up to 5 mm into the wood. As a preservative it is effective for exterior work above and below ground, but the thin layer makes it less useful in practice.

When to consider it

Tung oil seeps into the grain of the wood, giving it a perpetual wet look that highly accentualizes the grain of the wood, commonly referred to as "making the grain pop". Because of this, the color of the wood is slightly darkened, giving the wood a rich, warm color that is very pleasing.

Tung oil provides a relatively hard surface finish that, as long as the surface integrity is intact, provides a waterproof finish that is impervious to dust, alcohol, acetone (nail polish remover), and various acids such as fruit and vegetable acids (orange juice).

Tung oil has a proven history in exterior applications, both above and below the soil level. Though the dried oil is relatively hard, the finish it provides is not the most durable. Tung oil is usually chosen for its aesthetic appeal rather than its wearability.

In its pure form, tung oil is a non-toxic finish that is ideal for surfaces that are expected to come into contact with food. This includes wood cutting boards, salad bowls, salt and pepper mills and any other project imagineable.

Advantages

The following refers primarily to pure 100% tung oil. Please see the Variations section below for properties of those variations.

As long as the surface bearing the tung oil has not been damaged, tung oil provides the following benefits:

  • Waterproof (or at least highly water resistant on a well maintained surface)
  • Resistant to alcohol
  • Resistant to acetone (such as nail polish or remover)
  • Resistant to fruit acids and vegetable acids (such as orange juice)
  • Flexible. Oil finishes continue to protect as the wood expands and contract.
  • Accentuates the texture and grain of the wood
  • Easy to apply (rub on, rub off)
  • Very forgiving during application
  • Easy to re-apply if the original finish becomes worn or damaged
  • Non-toxic and food-safe (Pure form only! Be careful!)

Disadvantages

Tung oil produces a mildly disagreeable odor for a few days after it is applied. This odor lessens with time, however some find that it continues for quite some time afterwards. If the tung oil is to be coated with some other finish such as wax, shellac, lacquer or polyeurethane, this smell is no longer noticeable.

Tung oil takes time to dry. Today's high-tech woodworker is often in a hurry, but true tung oil takes its time to cure.

Pure tung oil has relatively poor penetration, and scratches that penetrate the finish can expose the bare wood beneath. This can be compensated for by adding up to 50% turpentine as a thinning agent to improve penetration on the first coat only. Subsequent coats should be done with un-thinned tung oil.

Pure tung oil is difficult to store. Depending on temperature, and exposure to light, the surface of the oil in the container will start to form a film or there will be gummy deposits around the container's edges. Once these symptoms appear, the entire container must be discarded, since the oil will no longer be able to cure properly if applied.

Variations

Please note that all these variations on tung oil go through processes that render the finish toxic. Only pure tung oil which has not been thinned can be considered non-toxic. Please be careful.

Tung Oil Finish (or Modified Tung Oil)

In an effort to give the consumer a product that produces a look of tung oil yet gives a harder and faster drying finish, many vendors are offering products with names such as "Tung Oil Finish". These products usually include metallic drying agents, thinners, or less expensive oil additives. These additives improve the penetration, hasten the drying time, and keeps the product cost reasonably low.

These products, while providing obvious benefit to the consumer, no longer behave or look like pure tung oil, and often these products contain only a small amount of tung oil and some do not contain any tung oil at all.

Polymerized Tung Oil

One of the main complaints of tung oil is the time it requires to properly cure once applied. Vendors have therefore produced a product called polymerized tung oil that has been through a cooking process to partially complete the molecular process that drying oils go through.

How to use it

As with any traditional product that has been used for many years, there will be many different opinions on how to use the product. What I will present here is the most simple and straightforward technique, and I will identify optional steps as such.

Precautions

Be careful with the handling and disposal of the rags used to apply tung oil. The oil itself is not a problem, however the solvents used to thin the tung oil are highly flammable and combustible. Allow rags to thoroughly dry on a non-flammable surface (such as a concrete block), or washed, or soaked with water before placing in the garbage. Solvents can generate heat through an exothermic reaction with the air (oxidation), and this reaction accelerates as the rags get hotter, and this has been known to start unintended fires.

Preparation

Since tung oil will highlight the grain and surface of the wood, it will also highlight any surface blemishes and scratches that are left behind. Therefore, take your time to prepare the wood surface before you start to apply the product.

Sanding sealers are usually used to fix problems with uneaven absorption of stains, however it is not required under penetrating oil finishes such as tung oil. If you do choose to use sanding sealers, it is recommended that you are sanding to 180/220 grit, and only move on to finer sanding once the sealer is applied.

Sand the surface using progressively finer sandpaper until you reach 220 grit (or optionally 320-dry grit) sandpaper. This will leave the surface smooth and there should be no visible scratch marks that remain.

Application

Use any clean, lint-free rag to apply tung oil. If you use a rag that has rough-cut edges, be sure to fold the rag so the rough edges are held in your hand and are not on the wood surface, since any loose wood fibres may catch the edge of your cloth and leave troublesome lint behind.

Apply a small amount of oil to your cloth (not the wood surface) and proceed to apply the oil to the work surface. Work from the least accessible areas (interior or underside) first and work towards the easiest sides. This will keep you from putting your elbow or forehead against a freshly oiled surface ;)

Apply by rubbing along the grain. Do not starve the wood, meaning you should not have to rub hard or often to get the surface wet, and if you are then re-oil your rag more often. Do not over saturate the surface either, meaning the surface should be wet but the surface should not have any standing puddles. If there are any puddles, remove them before continuing on to the next section of your project.

Approximately 1/2 hour after applying the oil, rub the project with a clean rag to remove any excess surface oil. Some people feel that a more vigorous buffing heats the oil and increases its penetration into the wood surface, however this          is an optional effort that may produce minimal returns at the expense of your elbow grease.

Let the project stand for at least 24 hours, or until dry. It may take weeks for a full curing/drying, but I am referring to the absorption of the oil by the wood surface. To obtain a super-smooth finished surface, rub the project surface with 0000 steel wool (or equivalent). The dust produced from this sanding should be a white powder, and if you get a gummy resin instead, wait another day before proceeding. Repeat the application of the oil, 1/2 rest period, and removal of the excess.

To get maximum protection, you should apply 3 coats of full-strength tung oil. You should also have sanded with 0000 steel wool the day after applying each coat.

As an optional step, you can use thin the first coat of tung oil by 50% or more using turpentine or naphtha. This will greatly increase its penetration into the wood surface, but you should still apply three coats of full-strength tung oil on          top. Please note that if you thin the first coat, the project will no longer be food-safe!

How do you know the surface is fully cured and has achieved its full hardness? If you push your fingers across the grain at various locations on your project, your fingers should slide smoothly and easily across the surface without any drag or grab. If there is any resistance, then the surface is not fully cured, and you should give it additional time before applying a different finish (such as shellac or paints).

Repairs

Repairs for projects finished with tung oil couldn't be simpler. It follows the same model as the initial application. Just use a clean rag and apply a small amount of tung oil to it. Rub the damaged area with the tung oil, let stand for 1/2 hour, then wipe off any excess. After allowing to dry overnight, lightly sand with 0000 steel wool. Repeat the process two more times and the project is ready for use again!

Removal

Before full curing/drying

If you want to remove the tung oil within 24 hours of application, you can flood the area with a tung oil thinner (such as turpentine, naptha, xylene, or Sutherland Welles Ltd.® Di-citrusol) and use #0000 steel wool to rub the surface. The tung oil will soften and become gummy, and you will need numerous clean pads and fresh solvent to successfully remove it.

Note that completely removing any trace of the tung oil may be impossible without removing the outer surface of the wood...

After full curing/drying

Sandpaper and elbow grease is your only option. The tung oil chemically bonds to the surface, which is usually considered a benefit. If you absolutely must remove the tung oil, you must remove the surface of the wood that it was bonded to.

Where to get it

Lee Valley Tools
Sutherland Welles Ltd®

Cleanup

Please see the Precautions section above for appropriate disposal of rags used to apply tung oil.

Tung oil is essentially an oil product, and can be cleaned from hands using soap/detergent and warm water. Thinner (turpentine or naptha) can be used to remove tung oil from surfaces that can not be properly washed.

References

Chemistry of oils during the drying/curing process
Food and Agriculture Organization of the United Nations - Minor Oil Crops - individual monographs

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