Woodworker's Journal 2011-35-Winter, страница 48

Working With Water-based Finishes

Water-based finishes offer many

benefits, and knowing their unique characteristics will ensure success.

Water-based finishes have taken the woodworking community by storm, and for good reason. They are similar to solvent-based finishes in durability, but smell less, give off fewer solvent fumes and are non-flammable. That's enough good news to make them worth looking into, but they do have a few quirks and drawbacks to consider.

The Nature of the Beast

The first thing to clarify is that water-based or waterborne finishes (the two terms are used interchangeably) are NOT water-soluble. A water-soluble finish would dissolve in water after it was dry and would not be waterproof. All water-based finishes are waterproof. In fact, the resins in water-based finishes are quite similar to those in standard solvent-based finishes.

Another misconception is that waterbornes are solvent free. Waterbornes have just enough solvent to form the film but are diluted with water to make them thin enough to spread. Where lacquer might be 75% solvent, water-base will have less than 10% solvent along with 60% water, and several additives to make the mixture work. These are very complicated formulas, and it is best not to thin them with water.

Over-applying is one of the top problems with water-based finishes. To prevent this, use a foam paint pad, as shown above.

Types of Waterbornes

Most water-based coatings are made using acrylic resin, polyurethane resin or a blend. The vast majority of coatings are a combination of the two resins, even though they may be called simply "polyurethane" or even "clear finish." Polyurethane resin is tougher and more durable, while acrylic resin adds clarity and good brushing and spraying qualities. Acrylic water-based coatings are about as durable as furniture lacquer, and water-based polyurethanes are about equal to oil-based polyurethane. The blends vary in durability, but most are tougher than lacquer and similar to polyurethane.

Most solvent-based finishes are slightly amber in color, but waterbornes are clear. Those

that contain a lot of polyurethane resin may have a slight blue-gray tint, but that will only be apparent when many coats are applied over very dark wood or stain. Waterbornes keep light woods blond and won't "yellow" white or light color stains. They also stay clear over time. Some are available in both interior and exterior formulations, and the ones designed for outdoor use will say so on the label.

Water-based coatings form a finish in two distinct stages: first water evaporation, and then solvent evaporation. After you spread the finish on the wood, the water starts to evaporate, leaving the solvents and the resin. This lets the mixture stick together to form a sort of brickwork of finish globules. The solvent evaporates last, leaving behind a dried layer of resin.

Drying Time

The coatings can be dry to the touch in as little as ten or fifteen minutes and feel hard enough to use in a day or two. A small amount of solvent stays behind, however, and evaporates very slowly. Until it is gone, the finish is not yet as tough as it will be once it is fully cured. For that reason, you should avoid using any cleaning chemicals or solvents on the finish for at least a month.

48 Working With Water-based Finishes