Woodworker's Journal 2011-35-Winter, страница 45
ture of two or more resins, usually acrylic and polyurethane. Acrylic resin has properties similar to lacquer, including its susceptibility to some strong solvents, chemicals and heat. It is added to the finish for two reasons: acrylic is less expensive than polyurethane resin, and it brings better brushing and spraying properties to the mixture. The combination of the two resins puts water-based polyurethane somewhere in between lacquer and oil-based polyurethane on the durability scale. Exactly where depends on how much of the mixture is acrylic resin and how much is polyurethane. Some brands are largely polyurethane, while others are mostly acrylic, so there can be significant differences from brand to brand. The question is, how much polyurethane is in your polyurethane? Drying Time This one goes to the water-based side of the scoreboard. One coat of oil-based polyurethane will take several hours before it is dry to the touch, but most water-based versions will get there in 10 minutes. The faster setup time means it is easier to get a dust-free finish with water-based polyurethane. Odor, Cleanup and Safety Once again, water-based comes out ahead. It emits less offensive odor and cleans up with soap and
The primary differences between oil-based and water-based polyurethanes relate to how they are made. Oil-based, whose proper name is "oil-modified urethane," is produced by reacting common finishing oils, like linseed oil, with a chemical that causes the oil to form larger molecules. A good rule of thumb is that larger molecules mean more durable finishes. The primary differences between oil-based and water-based polyurethanes relate to how they are made. Oil-based, whose proper name is "oil-modified urethane," is produced by reacting common finishing oils, like linseed oil, with a chemical that causes the oil to form larger molecules. A good rule of thumb is that larger molecules mean more durable finishes. water. Oil-based polyurethane smells more and requires mineral spirits for cleanup. Most oil-based polyurethanes are flammable, whereas none of the water-based ones are. Perhaps worse is that oily rags can be spontaneously combustible, so just in case, lay them out one layer thick to dry prior to disposing of them. Appearance Beauty is in the eye of the beholder, so this category has no definite winner, but there are obvious differences in how the two types look. Oil-based polyurethane is amber and will add subtle color to wood, while water-based is clear and adds no color at all. This is especially noticeable on light woods, like maple, which stay white under water-based polyurethane but get amber with oils. The flip side is that oils penetrate into wood better, resulting in greater chatoyance, or shimmer and depth, than you'll get with water-based. Oil-based polyurethane looks richer and more vibrant, especially on dark and highly colored woods, where water-based coatings can look pale or washed out. There are two other characteristics unique to water-based coatings that exacerbate appearance problems. Water-based polyurethane can bridge over 180-grit or coarser sanding scratches, leaving minute air spaces below the finish in the scratches. These can make the finish look pale and cloudy unless the raw wood has been sanded to 220-grit or finer. On some woods, such as poplar and oak, certain water-based coatings can draw extractives from the wood that react with the polyurethane, turning it slightly gray. You can get around both these problems by sealing the raw wood first with Zinsser SealCoat ". It provides an excellent sealer coat under water-based polyurethane and makes the wood look better to boot. Application Wiping on oil-based polyurethane will give you a thin, woody finish with no brush or spray marks, but continues on page 46... winter 2011 45 |
