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The two commonly available pure oil finishes that can be used on furniture and woodwork with decent results because they cure – that is, turn from a liquid to a soft solid – are linseed oil and tung oil. There are important differences between these two oils.
Raw linseed oil cures much too slowly to be practical, so use “boiled” linseed oil instead. It contains driers, which are catalysts to speed the introduction of oxygen and therefore the curing. It isn’t actually boiled.
You have to be careful with tung oil because many products are labeled “tung oil” when they are actually varnish thinned about half with mineral spirits (paint thinner). It makes no difference whether the varnish was made with tung oil; it’s still varnish. It dries hard and can be built up. You can identify these by reading the fine print on the label, which will list a thinner: “contains petroleum distillates,” “contains mineral spirits,” “contains aliphatic hydrocarbons,” which are different names for the same thing. Real tung oil is usually labeled 100% tung oil and never contains a thinner.
The primary differences between linseed oil and tung oil are as follows:
- Linseed oil “yellows” more than tung oil. That is, it turns more orange as it ages.
- Boiled linseed oil cures faster than tung oil, overnight in a warm room when all the excess is wiped off, as opposed to two or three days for tung oil. (Raw linseed oil cures much slower – weeks at a minimum – so raw linseed oil will remain sticky for a long time, even with the excess wiped off).
- Boiled linseed oil used as a finish can be made presentable with just two or three coats, sanding smooth after the first coat. Tung oil requires five or more coats, and you need to sand between each to remove the roughness.
- Tung oil is more water resistant than linseed oil because it has approximately three crosslinks between molecules instead of the slightly less than two for linseed oil. But because neither oil hardens well so neither can be built up thick, both are less water resistant than a built-up alkyd or polyurethane varnish, lacquer, shellac or water-based finish.
If you were to choose between using boiled linseed oil or tung oil for your finish, I would think you would almost always want to use boiled linseed oil. The increased water resistance you get with tung oil is too little to compensate for the increased time and effort required to get a presentable finish with tung oil. Think two-to-three weeks with five or more coats.
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