Tung oil

Overview

Tung oil comes from the seed of the tung tree and is a drying oil that hardens by polymerization. Wikipedia describes the result as a deep, almost wet look. It is the traditional oil for water-resistant, low-sheen work on tabletops, bowls and boat parts below the varnish. The catch is the label: Wikipedia notes that products sold as "tung oil finish" are not necessarily pure tung oil, and the term is also used for oil-varnish blends and wiping varnishes. Read the ingredients; "100% pure" is the term to look for.

Pros

• Cured pure tung gives better water resistance than linseed oil; makers claim it will not yellow as much (a manufacturer claim).
• Penetrates and leaves no film to chip; repairs blend in.
• Pure versions have no solvent and no metallic driers (Waterlox's statement for its product).
• Used by woodworkers for bowls and boards, subject to the food-contact caveats below.

Cons

• Very slow cure: days between coats, weeks to full cure; cold or damp rooms slow it more.
• Thin protection; the USDA Wood Handbook says drying oils like linseed and tung suit indoor use away from water and high humidity and that oil finishes soil more easily than film finishes.
• Easy to over-apply and get a gummy, wrinkled surface; "boiled" tung oil with excess cobalt drier was historically used for wrinkle finishes (Wikipedia).
• Can darken and look uneven on some woods without sealing.

Suitability

Good on White oak, European ash, Hard maple and the walnuts (Black walnut, European walnut) for tables and interior furniture. Weak choice for Teak and other very oily or resinous woods, where cure can fail; solvent-wipe and test. Not an exterior finish by itself.

Coats and application

Sand to 180 to 220. RMP recommends 3 to 5 coats. Thin the first coat with citrus or mineral spirits if the wood is dense (RMP sells a half-and-half tung and citrus-solvent version). Apply with a rag, soak for 15 to 30 minutes, then wipe off all excess; wait until the previous coat feels dry, typically a day or more, before the next. Rub lightly with 0000 steel wool or 400 paper between coats if grain lifts (Raising the grain). Liberon's schedule for its pure tung oil: dilute the first one or two coats with a little white spirit, let each coat penetrate 20 to 30 minutes, wipe off the excess, allow 24 hours between coats, finish with one or two undiluted coats (minimum 4 coats in all), wipe over again after 24 hours to remove oil that has surfaced, and leave a few days before use. Liberon quotes 8 to 10 m2 per litre. Coverage for RMP is about 50 sq ft per pint.

Compatibility

Can be topped with paste wax (Paste wax) after cure. Spar or oil-based varnish (Spar varnish) can follow a fully cured tung coat, but test for adhesion. Dewaxed shellac (Dewaxed shellac) is a barrier if you want to change finishes. Do not put water-based products or mineral-oil polishes over uncured oil. Compare Boiled and raw linseed oil, Polymerized linseed oil (stand oil) and Danish oil, which can be tung-based.

Care and repair

Re-oil dull areas with a thin coat after a wash with a mild soap. Heat rings and scratches can be spot-sanded and re-oiled. No stripping is needed unless you are changing finish; sand.

Safety and environment

Liberon's label warns that tung oil is extracted from nuts and may cause an allergic reaction. Rags soaked with tung or other drying oils can heat and ignite spontaneously. The USDA Wood Handbook says to immerse them in water and seal in a water-filled airtight metal container; see Oily rag safety.

Food contact: Waterlox claims its pure tung is "food-safe and FDA approved for food contact", and RMP says its pure tung is "food-contact safe when fully cured". Both are manufacturer statements, not regulator certifications of your finished piece. 21 CFR 175.300 lists chinawood (tung) oil among permitted drying oils for resinous and polymeric coatings, and says they may be raw, heat-bodied or blown, but the section governs coatings that act as a barrier on food-contact surfaces. The Wood Handbook says tung and other drying oils can seal butcher blocks and boards but may need several weeks to cure, and advises checking the label, manufacturer or FDA. See Food-safe finishes. Treat blends and "tung oil finish" products as not food-safe unless the label says otherwise.

Related

• Danish oil: tung-based blends exist under this name.
• Polymerized linseed oil (stand oil): a faster alternative.
• Walnut oil and other food oils: other oils for boards.
• Wipe-on polyurethane: faster, tougher wiped film.
• Oily rag safety: fire hazard.