Stagel: Water evaporates, forcing resin droplets I micelles I closer together.
Stage 2: Tail solvents evaporate, soft- Dry Film: Cloudiness disappears, ening mkeiles and allowing them to leaving clear resin color, coalesce into a continuous fUm.
^ter-Based Finishes by michael dresdner
CC "IVT" ater -based" has mX/ *>ccomc buzzword ▼ ▼ of finishing in the '90s, carrying with it reports that alternately praise or damn these relatively new coatings. But what is the real story behind water-based Finishes?
Kinder, Gentler Finishes
Water-based coatings have made their biggest impact because they have less of an odor than other finishes, clean up with water, arc safer to the user and the environment, and are non-flammable. (They are perhaps the only finishes that can be sprayed without an explosion-proof booth.)
These properties are the result of advances in chemistry that make it possible to mix the "resins," which form the coating on wood, with water. All finishes contain resins, and a solvent or "vehicle" that enables you to brush or spray the resins. In water-based finishes, the resins have been chemically altered to make them compatible with water. They are not dissolved. but are forced to coexist with water in a mixture called an "emulsion."
Emulsions of incompatible liquids are not uncommon. We're all familiar with milk (butterfat and water) and salad dressing (oil and vinegar), to name just two.
Like most emulsions, water-based finishes appear cloudy or milky when liquid, then take on the color of their resins when dry. In water-based finishes, the resins dry either clear (acrylic) or slightly bluish (mixture of polyurcthane and acrylic), rather than the familiar amber color of nitrocellulose lacquers or oil varnishes.
Water-based finishes are formulated to have properties of durability that are similar to their solvent-based counterparts. For example, you can expect products containing polyurcthane resins to have superior resistance to abrasion, cracking, and chemical or water damage. Those containing acry lic resins are generally clearer and more stain-resistant than the polyurcthancs.
Most water-based coatings form a film in the same way lacquer does— by the evaporation of solvents—rather than through the polymerization associated with oil varnishes. But with water-based coatings, this process occurs in two stages. (See drawing.)
First, the water evaporates, compressing the suspended droplets of resin, called "micelles," on the wood surface. Then the hydrocarbon solvent, or "tail solvent," remaining in the mixture softens these micelles, allowing them to coalesce into a film as the tail solvent evaporates.
Even though water-based finishes contain only small amounts of volatile hydrocarbon solvents, you should use a fan for ventilation and wear a vapor mask when applying them, as with any other finishing material.
Getting It On
It isn't difficult to produce an excellent finish using water-based materials, but here are a few pointers to make it even easier. /;/ general...
• Don't let the finish freeze. Remember, it contains water.
• Use it when the temperature is 70°-90°F with 40%-70% relative humidity. Water-based coatings arc sensitive to their environment as well as good to it.
• Apply light, thin coats. Thinner is better with all air-drying finishes, but here it Is an absolute must. Thick coats will not only run and sag. they will also dry p<x>rly and may stay white or hazy.
• Don't apply more than two coats per day. Allow drying time between coats. Follow pixxJuct recommendatioas.
• Scuff' the surface lightly between coats with fine sandpaper or a Scotch-Brite pad to aid adhesion of subsequent coats. Avoid steel wool. It may contain oils and can leave invisible bits of metal that will turn into invisible rust when you apply the next coat.
• Don't use a tack rag. It can leave an oily residue. Instead, use a cotton cloth dampened with water. It's cheaper tluui a tack rag and won't contaminate the surface.
• The residue from stcaratcd (self-lubricating) sandpaper can cause "fish-eyes" or other flow-out problems. If you use stcaratcd sandpaper, remove the dust with a damp cloth.
For spray application...
• Use a fluid tip with a small aperture (approx. 1 mm or 0.04() in.) to help you spray a light coat.
• L'sc an HVLP gun or a low-pres-sure setting (about 25 psi) on your compressed-air gun. Hither one will do a good job.
• If you are using a compressed-air gun, your air supply should be clean, oil-free, and above 70°F.
For band application...
• Choose a soft nylon or nylon/polyester-blend brush. Natural bristles will splay in water.
• Make sure your brush has tipped or tapered bristles. Flagged bristles.
those with split ends, may cause foaming, leaving air bubbles in your finish. (For more on brushes, see aw »26, page 62.)
• For lar^e, flat surfaces, try a short-nap paint pad (available at paint stores).
• For small objects, you can use a lint-free rag or a printer's litho pad to apply finish.
• Whether you use a brush, a pad, or a rag, flow the material on in one direction as you would shellac or lacquer. Back and-forth or scrubbing motions can leave air bubbles in the finish.
Of all the brands I've used, I give the highest marks to Carver Tripp, .Vlinwax, McCloskey, and Zar. Carver Tripp is the easiest of the four to spray, while McCloskey has a slightly warmer color than the others.
Water-based finishes have come a long way since their introduction several years ago. You should give diem a try.
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