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Expert's Corner — seal

TIP: Polyurethane and Sealers

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Oil-based polyurethane is a very durable and hard-curing finish. It bonds well to itself, especially if each coat is sanded a little after it has dried well enough so it powders. This creates fine scratches, which improve the bonding of the next coat. It’s a good idea to do this fine sanding between coats anyway to remove dust nibs. But polyurethane doesn’t bond so well over finishes marketed as sealers, especially over sanding sealer. This sealer is good for use under non-polyurethane varnishes because regular alkyd varnishes gum up sandpaper. So to speed production, a sanding sealer can be used...

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TIP: Finishing Over Pine Knots

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The resin in pine knots contains solvents that will bleed into and through most paints and finishes. This can cause the paint or finish to remain sticky, and it can cause the orange color to bleed through as shown in the accompanying picture of white latex paint applied over pine. There are two types of products on the market that will block this resin: white pigmented primers and clear shellac. The most well known white primers are Kilz and BIN. The best clear shellac to use is Zinsser SealCoat because it has very little color and a longer shelf life...

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TIP: A Solution to Overspray Problems

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If you are spraying without adequate exhaust, you may experience finish in the form of fine dust settling on your work, sticking to it, and causing it to feel rough, almost like fine sandpaper. This isn’t a problem until your last coat because you can easily sand out the rough feel. But you can’t sand your final coat without then going to the trouble of rubbing the entire surface with finer-and-finer-grit abrasives until you create a pleasing even sheen. If you are spraying shellac or lacquer, there is a solution to the overspray problem. After spraying your final coat, spray...

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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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