tung oil


Can I use tung oil under spar urethane for an outdoor sign? The wood is mahogany and I would like to bring out the natural color and grain. Would boiled linseed oil be a better choice under the spar?
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wrote:

As long as you let it cure properly, I'd think either would work. In my experience, tung cures faster than BLO.
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The stories surounding Tung Oil are probably part and parsel of some of the largest urban myths circulating in woodworking today. It was brought into Europe from the far east and used because it dried harder and was a bit more water resistant than the boiled linseed oil that the poor old Europeans were using. Once the poor old europeans developed resin based varnish, they quit using it. Linseed oil is no longer boiled to crosslink the molecules, but instead has metalic dryer added to it. Pure tung oil does not and in the right climate can take days if not weeks to fully cure. Most stuff that is called tung oil (I.E. Fornbey's) is really wiping varnish that has polyurethane along with the tung oil. To the wood, oil is oil and it is the oil that brings out the depth and color of the wood. Having said that, varnish has oil in it. Its what makes the resulting resin film (be it traditional phenolic or polyurethane) flexible enough to stay on wood which we all know moves. Spar varnish is a 'long oil varish' which means it has a lot of oil in it that results in a flexible film when it cures. It also UV inhibitors, go figure - outdoors.
In my experience, a first coat of spar urethane thinned about 50% will yeild the same affect as screwing around with linseed or tung oil. But if you must, who am I to say.

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Why? Spar varnishes are long-oil types anyway. You'll get what you're after - briefly.
As Max says, thinning coat one and possibly two will get a bit better penetration.
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Lee wrote:

Why not just skip the spar and use Penofin or Olympic Maximum oil sealer?
Barry
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