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Expert's Corner — paint

Wetting to Predict Blotching

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Wetting to Predict Blotching

You can usually get a pretty good idea whether or not the wood you are using will blotch when a stain or finish is applied by wetting the wood.You can use any liquid, but water will raise the grain causing you to have to sand more. Mineral spirits (paint thinner) works well except if you intend to apply a water-based finish. Some residue oiliness may remain and cause the finish to fish eye—that is, bunch up into ridges rather than level out.Denatured alcohol would be better for this situation because it will totally evaporate. But it evaporates quickly, so you...

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Finishing Tip by Bob Flexner: Testing for Shellac

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Finishing Tip by Bob Flexner: Testing for Shellac

Almost all furniture and woodwork finished between the 1820s and 1920s was finished with shellac. But if you want to test to be sure, here’s the way to do it. Put a little denatured alcohol on your finger and dab it onto an inconspicuous area of the finish, as shown in the accompanying picture. If the surface gets sticky or if the finish comes off on your finger, the finish is shellac. Shellac dissolves in alcohol, so you could use the alcohol to strip the finish if that is your intention, instead of using paint stripper.

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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: Thinning Polyurethane with Naphtha

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Naphtha dries (evaporates) much faster than mineral spirits (paint thinner). This leads many to believe that thinning polyurethane with naphtha will make it dry faster. This is not entirely the case. Like all varnishes, oil-based polyurethane dries in two steps. The first is evaporation of the thinner. The second, and much longer step, is by the crosslinking brought about by the introduction of oxygen from the air. When you apply polyurethane, you notice that it stays wet on the surface for a short time as the thinner evaporates. Then the finish goes into a tacky or sticky stage for an...

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TIP: Patina

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TIP: Patina

Patina is primarily the mellowing and color change that occurs in wood over time due to oxidation from exposure to air and bleaching from exposure to light. Secondarily, patina is the dings, scratches, rubs, etc., that give old furniture character. The mid-nineteenth-century cylinder roll-top desk in the accompanying picture has patina, primarily the bleaching of the mahogany and also the bleaching of the plaster-of-Paris used to fill the pores. The desk has never been refinished, but the finish has been renewed with French polishing. Adding finish on top doesn’t change the coloring of the wood underneath. Patina is highly valued...

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