Finishing is hard enough even without the mislabeling that is so prevalent on the part of many manufacturers. The mislabeling makes it difficult for us to know what we are buying and using.
The accompanying picture shows dried puddles of two commonly available products both labeled “tung oil.” These two products could hardly be more different.
The tung oil on the left is real tung oil, the oil produced from pressing the nuts from tung trees, which are native to China. A puddle of the oil dries wrinkled, and it never really hardens. You can always scrape it off with your fingernail. As a result, you need to wipe off all the excess after applying each coat or the resulting finish won’t be functional. With real tung oil you can’t get a build.
The “tung oil” on the right is actually varnish, which has been thinned with mineral spirits (paint thinner) so it is easy to wipe on the wood. But it dries hard, so it can be left thick on the wood and can be built up with several coats for better resistance against water penetration.
You might think: well, maybe the finish on the right was made with tung oil, so that’s the reason the manufacturer labels it “tung oil.” But this isn’t the case. It’s made with modified soybean (soya) oil. You can tell this (even if the manufacturer doesn’t volunteer the information) by the lack of yellowing in the finish as it ages.
Varnishes made with linseed oil or tung oil yellow significantly, just as the real tung oil on the left has. The reason modified soybean oil is used in most varnishes (including polyurethane varnishes) is because it yellows very little. It is more “lightfast.”