Tung oil: why and how to use it

The author in his workshop, shown applying tung oil to a test board.

Words and photos: Peter Spaulding

Shortly after moving from Tennessee to Southern California, I needed to resupply on my favourite finish. The closest store to carry it was an hour and a half away, but off I went. It was only upon arrival that I made a horrible discovery – some clean-air legislation enacted by my new state had removed it (and almost every other finish I’d ever used) from the shelves. I was displeased, to say the least, and not just because of the wasted drive. However, I wound up buying a bottle of tung oil – sans the also recommended citrus solvent– and thus began an entirely new finishing journey.

Misconceptions

Problems with tung oil mainly arise from poor technique, worsened by what you see on the internet. Pour on, mop around, wait an hour or two and then assess whether to do a second coat...right? Not at all. If it ever cures, it won’t be pretty. Additional coats will compound the problem. No matter how gorgeous and ‘grain popping’ it seems at first, a poor tung oil job only looks good while it’s still wet.

Product confusion is partly to blame here – tung oil and ‘tung oil finish’ are not the same thing. In fact, if not specifically labelled ‘pure’ or ‘100%’, it’s likely the supposed tung oil has been chemically modified or is altogether absent, and will therefore not behave as tung oil.

Why use it?

In the realm of eco- and health-friendly finishing, tung oil is a champ – few other finishes perform so well in so many ways. Even when set against synthetic finishes, in my opinion tung oil holds its own. This performance superiority arises from its unique fatty acid composition – uniquely high in alpha-eleostearic acid, and uniquely low in the fatty acids that make up other drying oils – giving it a combination of properties few finishes have.

It doesn’t mould, turn amber, or go rancid

Natural or synthetic, food-safe or not, most finishes face one or more of these issues. Most oil-based and synthetic finishes increasingly discolour as they age, while many pure oil finishes run the risk of becoming rancid before use. Worst of all, some finishes allow mould to grow where damp or wet conditions exist, however tung resists this. Even linseed oil, unless of the highest quality Swedish origin variety, faces all three problems – in my experience, tung oil does not.

The finish on this small wall cabinet was achieved with 8–9 very lights coats of tung oil applied over a week or so. The sheen on its surfaces changes when light hits it in different ways.

It’s waterproof

It may seem like all furniture oils and oil/wax blends are too (after all, oil and water don’t mix), but they’re not really. The thing is, drying oils don’t actually dry and they don’t remain as oil. They polymerise. There’s no such thing as ‘dry oil’, but there is ‘cured’ or ‘polymerised’ oil.

In simple terms, polymerisation causes these oils to form irreversible bonds, converting them from oil to a cross- linked solid (think of it as a microscopic chain-link fence). Some link tighter than others though, and none link tightly enough to stop water molecules from popping through – except for tung oil. With enough coats to fill the pores and scratches left in the wood, liquid water doesn’t get through. Tung does this so well it was used for centuries in China to waterproof the hulls of boats. Nowadays, you can still use tung oil on a boat, but it’s particularly useful on woodenware and outdoor furniture too.

Any good finish begins with good surface prep. Tung oil will highlight rather than hide, so surface prep is important.

It’s non-allergenic

Speaking of China, ‘China nut’ or tung oil actually comes from the seeds of a berry. Though this berry of the Vernicia fordii tree outwardly resembles a nut, tung oil is said to be safe for people with nut allergies.

It’s safe

You shouldn’t drink it or cook with it, but it isn’t bad for the environment, or unhealthy for you to apply. It’s safe for the people using your woodenware, safe for the children chewing the edge of the table, and safe for the dog eating the legs of the bed. And while I’m thinking of those awful scratches in that beautiful bed.

It’s repairable

Unlike those inevitable scratches in a hard film finish, scratches in most oil finishes are less visible and far easier to repair. This is partly because oil cures a little softer by comparison. But, again setting it above its companions, tung oil isn’t too soft either. It’s the Goldilocks of hardness; most cases will rarely require more than rubbing a tiny bit of oil into the affected area.

Only rare circumstances will call for the need to strip or sand away a well-done tung oil finish – and that brings me to one of the most commonly misunderstood things about it.

Small squares cut from an old cotton T-shirt do the trick. A small rag is best, because it helps to prevent over-saturating the workpiece with oil.

You don’t need to sand

Not before, between or after coats. A lot of sources will say you do, but I’ve found it’s just not the case. Because I prep most surfaces these days with a blade, sanding has become rare in my shop. Tung oil doesn’t require it for adhesion, smoothness, clarity, or gloss (tung can be built to a gloss/semi-gloss too).

That said, if your sheared surface isn’t where you want it to be (or if you just prefer sanding), I recommend bringing the base surface to 600 grit or higher. This is again contrary to many claims, but the more sheared, polished, or burnished the surface, the better it is for tung. However you get there, once the first coat goes on, you don’t need to sand again.

Thin, not thinned

With surface-prep paving the way, the biggest key to success is to apply tung oil thinly, not thinned. Remember I’m still using the same bottle from two years ago? And I’ve already mentioned you don’t need solvent either, because tung oil doesn’t cure through evaporation but instead depends on heat, UV light and oxygen to cure.

Tung oil doesn’t discolour with time, allowing the character of the wood to shine through. The tung oil finish on this chair is satin and gloss at the same time, capable of reflecting the image of the back spindles off the seat.

You only need a small rag to be just wet enough without being able to squeeze oil from it. That’s it. Nothing else. I prefer to use roughly sized 60mm squares cut from an old cotton T-shirt. As well as being careful not to over- saturate the rag or the wood, it’s best to wipe on, wipe off, as previously unnoticed fingerprints and splotches can cure cloudy if you don’t. A gentle wipe with a dry cloth of the same material will do.

To give a better idea of how little oil is needed, take a recently completed wall cabinet as example. With 8–9 coats applied over 7–8 days, I dabbed the oil bottle against my tiny rag no more than 5–6 times, using less than an ounce of oil in total. The cabinet now has a satin glow in indirect light, and a diffuse reflectivity when the light shines directly on any plane.

Some of you may wonder how 8–9 coats can be applied with 5–6 dabs of oil. The simple answer is that I don’t wash or toss my rags between projects. Instead, I keep them sealed in a glass jar in the refrigerator until needed again. Though unlikely, should a fire start, it would soon starve of oxygen in the jar and go out without causing damage. In this way, I’m able to use the same rag for a month or two at a time. There’s often enough oil left in it to do repeat coats without recharging the rag.

For a faster cure

The second key to a great pure tung finish is probably my favourite – we can make it cure faster. By keeping those coats thin and supplying more of what it needs, it can cure harder and brighter too. With the right conditions – I like 50–75°C, low humidity, and the UV equivalent of a bright, sunny day – tung oil can be sufficiently ready for recoat or service in as little as five or six hours. You can take advantage of naturally hot, dry, sunny days, or make an oil-curing kiln to successfully and quickly cure tung oil no matter what the weather is.

Tung oil imparts a warmth and shine difficult to replicate with even the most modern finishes. Because it's both waterproof and food safe, woodenware is a great candidate for a tung oil finish.

Beyond tung

That failed trip to buy my former favourite finish eventually led me to discover other natural finishes. Tung oil is unique, but it’s not alone. From an incredible natural lacquer made of sap, to paint made with milk and ink made from natural glue, to a fermented juice that weatherproofs wood and can make paper strong enough to be used for clothes, the natural world of finishing is ancient, wide, and astoundingly relevant. I’d say it was worth the trip.

Peter Spaulding is a self- taught designer maker, working from his small home workshop in the desert of southern California. He began his career in woodwork building houses, but now spends most of his time making unique and varied pieces ranging from furniture and boxes to small sculptures, vases, vessels, platters, and spoons. Learn more at Instagram @Iwao_wood.and.art