by Jeff Jewitt
Originally published in American Lutherie #90, 2007
■ Eric Nicholson of Northern Ireland asks:
Woodworker magazine published an article on Stewart Adamson’s work regarding Tru-Oil finishes. His method involved a considerable amount of work with Micro-mesh during the process. I tried his method on my own guitars, both classical and steel string, with very satisfying results. Tru-Oil gives an attractive semigloss finish on all woods, but Adamson also goes on to say that for those that like a gloss finish, a gloss tung oil is now available only in the USA. Does anyone know of this gloss tung oil and where it is available in the States, or if it is now available in the UK?
Jeff Jewett of Homestead Finishing in Cleveland Ohio responds:
Tru-Oil is not pure tung oil. It’s a mixture of linseed oil, mineral spirits, and, according to the manufacturer, “modified oil.” It’s arguable that no tung oil is even used in this stuff. When finish chemists talk about “modified oil” as an ingredient they typically mean either chemical modification or heat modification. Chemically modifying a drying oil involves reacting it with chemicals and heat to form what are known as alkyds, which are used in varnish manufacture. Heat-treated oils basically jump start the drying/curing process. Pure heat-treated oils are used all the time in making finishes, but they rarely show up as “finishes.” The only pure heat-treated linseed oil I know of is Tried and True “Varnish Oil.” Pure heat-treated tung oil is sold in this country under the Sutherland Welles brand (www.sutherlandwelles.com). So I would be tempted to say that Tru-Oil is a mix of linseed oil and some sort of modified linseed or tung oil. Because it has a high oil content, it is not possible to produce a very glossy finish. High oil (also known as long oil) products tend to form microscopic wrinkles at the surface as they cure.
There is one way to heat-modify pure tung oil so that it will form a glossy film. This involves careful and controlled heating of the oil to about 450°F. Very few people can cook tung oil properly nowadays, but you can buy it under the Sutherland Welles brand “Original Formula High Lustre.” I don’t know if they will ship overseas. It’s not an easy finish to do, and I’m not sure that it will match the gloss of a French polish, but at least you know it is real tung oil, along with solvent and some driers.
