Woodworker's Journal 2011-35-Winter, страница 50
lightly between each subsequent coat anyway. The low solvent content means the finish can't redis-solve the coat below it, and a light sanding with 320-grit sandpaper will both smooth the surface and help increase adhesion. Avoid the white/gray self-lubricated sandpaper, since it can create fisheyes in the next coat. Instead, use the gold self-lubricated paper from 3M. It is designed for sanding waterbornes. Brushing You can brush, spray, roll, sponge or pad on water-based finishes, but some tools and techniques work better than others. For large, flat surfaces, I find that a paint pad is ideal. It allows you to cover large areas quickly and, because the nap is so short, it makes it more difficult to flood on too much finish. You'll get fewer air bubbles in the finish (called "foaming") and a much smoother end result. Start by stirring die finish well, then strain it through a paint filter or old nylon stockings. Water-based coatings sometimes form "gel specks," which are tiny globs of coagulated finish that look like pale tadpoles. Straining will eliminate them. Pour some finish out into a flat pan, dip the pad, then scrape off some excess so that the pad is wet, but not dripping. Stroke the pad over the wood, then go back over it ONCE, immediately to blend it out. After that, leave it alone. Going back over it again will only make it worse. For those areas where a flat pad won't work, switch to a nylon or "Taklon" brush. With all water-based materials, you must use synthetic bristle brushes, since natural bristles will "splay" and become soft and unruly in For those times when you simply can't get rid of those tiny air bubbles in the finish, try adding about one ounce of half and half cream per quart of coating. water. Of the synthetic bristles available, nylon is softest and will leave the fewest brush marks. Pre-wet the brush by soaking it in water for a minute or two, then shake out all the excess. Dip the bristles about a third of the way into a pan, and brush smoothly in the direction of the grain. You can go back over it immediately to smooth it out, but only once. Brush carefully; flipping the bristles through the wet finish briskly will create air bubbles. Don't brush back and forth, and don't go back over the finish after it starts to set up. The stuff dries very fast, and rebrushing or scrubbing will create foam and make the surface rougher. For those times when you simply can't get rid of those tiny air bubbles in the finish, add about one ounce of half and half cream per quart of coating. It will act as a defoamer, eliminating bubbles. Spraying Water-based coatings spray much like other materials, with a few minor differences. Starting with the gun itself, make sure the tip and all fluid passages are stainless steel or plastic, since plain steel will rust. I find that a smaller tip size works better — 1 mm to 1.3 mm. As with brushing, spray lighter coats than you are used to spraying with solvent-based finishes. Nitrocellulose lacquer is usually sprayed at well below 20% solids, while waterbornes are typically 30% or higher. One coat of water-based will be equivalent to one or two coats of lacquer. It's quite easy to get runs and drips on vertical surfaces, and spraying less will help with that, too. Don't worry if it looks a bit pebbly, since it will usually flow out by morning. Keep the gun tip about 8" from the wood to avoid overspray, and stick to one or two coats per day to allow plenty of drying time between coats. If you must thin the finish, I recommend a small amount of Flood Floetrol® rather than water. It will slow down the drying time and make it lay out more smoothly. Even though they aren't flammable, water-based coatings contain solvents, so you still need a spray booth and a ventilation fan. You may be able to use a more gentle fan, since there are fewer solvents. A strong flow of cool air over the wet finish can chill the surface and prevent it from flowing out smoothly. Cleanup If you've ever tried to clean a brush or spray gun using soap and water, you probably noticed that instead of dissolving, the finish turned into a gooey mass. That's because water-based finishes tend to coagulate if too much water is added to them — another good reason to avoid thinning the stuff. Instead, add some household ammonia to the cleanup water. It will help break up the finish and let you scrub the brush or gun clean. Incidentally, that ammonia trick also works when cleaning up water-based finishes' "first cousin": latex paint. & _S 50 Working With Water-based Finishes |
