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Expert's Corner — linseed

TIP: Tung Oil and Varnish Sold as Tung Oil

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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...

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TIP: Linseed Oil Gets Darker in the Dark

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Linseed oil (both raw and boiled) darkens in the absence of light. That is, it darkens in the opposite conditions than those affecting most finishes and woods. These usually darken when exposed to light, especially high UV sunlight or fluorescent light. Take a look at the accompanying photo of two cans of linseed oil in the process of going through their darkening stages. The can on the left shows fresh overspill, the slightly yellow color you expect from linseed oil when you are using it. The can on the right has been stored in a dark cabinet for several years,...

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TIP: Linseed Oil and Polyurethane

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I’ve heard it suggested that linseed oil (raw or boiled) should be applied to wood floors before applying oil-based polyurethane. Various reasons are given: seal the wood, establish a better bond, or add color. Doing this is risky, however. Raw linseed oil can take months to dry. Boiled linseed oil can take a week or longer deep in the large pores of oak. If you apply polyurethane before the linseed oil is thoroughly dry, the linseed oil and polyurethane will mix right on the wood and create an oil/varnish blend, which will never get hard. It would be like brushing...

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TIP: Oil and Spontaneous Combustion

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Drying oils, especially linseed oil, are the only finishing materials that spontaneously combust. Solvents don’t spontaneously combust, paint strippers (including paint or finish residue) don’t spontaneously combust, and no type of varnish spontaneously combusts. It’s not totally clear whether 100% tung oil can spontaneously combust, so treat it like it does. As linseed oil dries, it generates heat as a byproduct. If you wad up linseed-oil rags or pile them on top of each other, the heat generated in the middle can’t dissipate. It builds up until it reaches the flash point of the cloth and it bursts into flame....

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