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Expert's Corner — drying

TIP: Spraying Lacquer Over a Paint or Finish

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There are risks to spraying any type of solvent lacquer over any existing, and older, paint or finish. The problem is the lacquer thinner in the lacquer. A wet application can cause many paints and finishes to wrinkle or blister, even an old coat of lacquer itself. The two easiest ways to avoid problems are to spray several light (almost dust) coats of lacquer to get a bit of a build before applying wet coats, or to apply a coat of shellac before spraying the lacquer. Both methods will create a barrier to keep the existing coating from being excessively...

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Water Warps Wood Opposite From What You May Think

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Water causes wood to swell, so most people think that wetting one side and not the other will cause the wetted side to bow – that is, increase in width so the center is higher than the edges. If the wood is thin enough, this will be the case initially. But the overall swelling or shrinking after many wettings and dryings out, no matter the thickness of the wood, will be the opposite. The wetted side will shrink and the wood, or boards, will cup. The four accompanying pictures show examples of this. With a little thought, you will most...

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TIP: Basic Understanding of Solvents

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Following is a basic understanding of the common solvents available in paint stores and home centers. Mineral spirits (paint thinner) and naphtha dilute and clean up oils and varnishes, including oil-based polyurethane varnish. Neither of these solvents damage any fully dried finish, so you can safely use them for cleaning—that is, removing grease or wax. Denatured alcohol thins and cleans up shellac. This solvent will damage a dried shellac finish almost instantly and lacquer and water-based finish fairly quickly, so be very careful if you use alcohol for cleaning. Lacquer thinner and acetone thin and clean up all solvent-based lacquer...

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TIP: Wash Off Stripper Wax

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Directions on cans of paint-and-varnish remover instruct to “neutralize” the stripper as a final step. This is misleading and often leads to finishing problems. The instruction is misleading because there is nothing in paint strippers that needs to be neutralized. “Neutralizing” refers to acids and bases, not solvents. What needs to be done with all paint strippers sold in metal cans is remove the wax they contain. Manufacturers add wax to these products to retard evaporation so the stripper remains in contact with the paint or finish longer. This wax will retard the drying and weaken the bonding of most...

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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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