Woodworker's Journal 2011-35-Winter, страница 39
The author prefers to flood sealer on liberally by hand (left), then wipe off the excess with shop towels. Wear gloves for this procedure. Stearated sealer builds up quickly and sands easily — although it creates a lot of dust. One or two coats are sufficient. the deeper pores shrinks proportionally more, causing the pores to show up again as depressions in a matter of months. Universal Sealer Different types of sealers are often specifically formulated for particular coatings and tasks. For instance, both vinyl and ure-thane sealers increase adhesion on some substrates, add moisture resistance and block antioxidants. But choosing a sealer for your everyday work need not be a confusing task. Fortunately, there is a universal sealer that will do everything on the list above, and it is compatible with every wood and between all coatings. Made by Zinsser, SealCoat" Universal Sanding Sealer is a modified dewaxed shellac with a long shelf life. (Freshly mixed dewaxed shellac is an appropriate substitute.) When to Use Sealer Strictly speaking, any finish that forms a film on wood can be used as a sealer. Some coatings are so good at this task by themselves that they are called "self-sealing" finishes. Other finishes are not, and they benefit from special sealers. Shellac and oil-based finishes (including Danish oil, varnish and polyurethane) work so well by themselves that they do not require any special sealer under them. Some finishers prefer to thin the first coat of these materials to make them dry quicker or sand easier, but that is strictly a personal choice. Lacquer and water-based coatings, on the other hand, work better over sealer. The right sealer will lock in contaminating oils and waxes, reduce the number of coats needed by preventing excessive absorption, improve adhesion and reduce grain raising under water-based coatings. The wood also plays a part. With very dense woods, such as rock maple, you can usually omit the sealer. However, spongy or absorbent woods like poplar, red alder and most softwoods, can benefit greatly from sealer, especially under lacquer. The sealer coat envelopes the porous wood, preventing the first few coats of lacquer from being excessively absorbed. Some problem woods, like rosewood and cocobolo, contain antioxidants that prevent certain finishes from curing. These need sealer under oil-based coatings, but not necessarily under lacquer or shellac. Fortunately, it doesn't hurt to use the correct sealer, so when in doubt, err on the side of safety. Applying Sealer You can apply sealer as you would any coating, with a brush, gun or pad. The problem is that after one coat, end grain and spongy areas may still be "hungry" and insufficiently sealed, while denser flat grain areas are starting to build up too much coating. I prefer to flood the sealer on liberally by hand, using a nylon abrasive pad as an applicator, then immediately wipe it off with paper shop towels while it is still wet. Wear gloves and work small areas at a time so the sealer does not dry before you wipe it off. The advantage of this method is that it allows end grain to absorb as much sealer as it can, but wipes any extra off flat grain surfaces that tend to absorb less. Once the sealer is dry, the entire piece is uniformly sealed, and the next coat of finish will lay out the same in all areas. / The author's favorite sealer, Zinsser's SealCoat — a long-lived de-esterified shellac, works for all sealing applications. WINTER 2011 39 |
