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Expert's Corner — heat

How to Choose a Finish: Part II

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How to Choose a Finish: Part II

For an overview of choosing a finish please refer to “How to Choose a Finish: Part I.” To better understand finishes and their differences, it’s very helpful to put them into categories by the way they cure. You may think the resins—polyurethane, alkyd, acrylic, etc.—make the big difference in finishes, but they don’t. Consider, for example, that polyurethane resin is used in varnish, water-based finish, two-part finish and in some lacquers. If you have used any two of these finishes, you know they are very different. The curing process the finish goes through is far more significant for understanding each...

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Polyurethane Won’t Dry

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If you experience oil-based polyurethane not drying well, it’s not likely that it’s bad polyurethane. It’s more likely that the wood you’re finishing contains a natural oil or you have applied an oil to the wood and the oil hasn’t dried. In both cases we’re talking only about the first coat of polyurethane. After the first coat has dried, there shouldn’t be any further drying problems. Most exotic woods (woods from jungle areas), with the exception of mahogany, contain a natural resin that is very oily. You can feel the oiliness. The mineral-spirits solvent in the polyurethane is also the...

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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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