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Expert's Corner — polyurethane

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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Thin All You Want

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You often see cans of solvent finishes such as varnish, polyurethane and lacquer with instructions not to thin them. Manufacturers include these instructions in order to comply with VOC laws in some areas of the country. Adding thinner could take the finish out of compliance with the local or state laws, and manufacturers might be breaking the law if they advocated thinning. But you can’t do any harm to these finishes by adding thinner. In fact, you can add all the thinner you want, even 99 percent, without causing any harm. You’ll just get a thinner build with each coat,...

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This is Not Rocket Science

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This is Not Rocket Science

Back in the 1970s and 80s, it was common to see explanations of the difference between Danish oil and wiping varnish described as that between oil and resin-fortified oil. I couldn’t make sense of this explanation because the term “resin” is so vague. It commonly refers to fossilized tree saps such as rosin, copal and amber, and sometimes to synthetic alkyd and polyurethane. So, is one of these resins just added to the oil to “fortify” it, or is something else done? A further confusion is that linseed oil and tung oil are, themselves, sometimes classified as resins. So, is...

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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: Repairing Sand-Throughs on Edges

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The easy way to repair sand- and rub-throughs on sharp edges is with a marker of a similar color to that of the stain used. These markers are widely available. If you sand through an edge between coats of finish, simply color in the sand-throughs and apply another coat of finish, to keep the marker color from being rubbed off. If you rub through an edge on already finished furniture, woodwork or cabinets, it will be more difficult to apply a protective finish after replacing the color. The easiest methods would be to wipe just the edge with wiping varnish,...

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