Finish Well with Waterbornes
Waterborne finishes overcome old objections
Today’s water-based topcoats apply easier and look better than ever.
Looking for a crystal-clear finish that’s durable, easy to apply, and doesn’t fill your shop with fumes? Waterborne (WB) clear wood finish may be the ticket.
Early WB finishes proved troublesome to home woodworkers, often due to their unfamiliarity. But manufacturers kept improving their products to meet users’ expectations. Now those old objections—they add moisture to the wood, don’t resist water and dampness when dry, mar easily, and impart a hazy tint to the wood—no longer hold water.
Note: The water in waterborne finishes evaporates quickly after application; it doesn’t soak into the wood, so it doesn’t increase moisture content.
Chemistry makes it clear
Waterborne finishes (also referred to as water-based) behave like traditional polyurethanes—a solvent evaporates and leaves a tough film of interlocked resin particles— but the chemistry is more complex with WB finishes because the resins are not dissolved directly into the water.
The finishes work well for interior and furniture finishing, and, generally, equal the durability of traditional products. They resist water, but heat can mar the surface. Some finishes are susceptible to solvents, which are usually specified on the label.
Pluses and minuses
The absence of VOC (volatile organic compounds) fumes leads the list of advantages for WB finishes. Hazardous from health and fire-safety viewpoints, VOCs found in oil-based and lacquer products make finishing risky in home workshops that lack adequate ventilation. WB finishes, on the other hand, prove safer to apply, even in a basement workshop in winter. And you can clean up with soap and water [Photo A].
Generally WB finishes dry more quickly than oil-based polyurethanes and dry as close to crystal clear as any finish available, adding only slight coloration to the wood [Photo B]. That clarity also counts as a drawback: Many woodworkers value the warm amber tone that oil-based finishes create.
Note: Even though there are no VOC fumes, it’s still a good idea to provide adequate ventilation when appying WB finishes indoors.
Tip! Add traditional finish amber coloration by applying a coat of shellac before finishing with a clear waterborne product.
Using waterborne finishes
Even though a larger can may be more economical, buy just enough finish for your project. That way, you won’t have to store so much leftover material that may degrade and have to be discarded. Before applying waterborne finish over oil-base stain, allow the stain to dry at least 48 hours. A coat of shellac over the stain improves finish adhesion.
Most dealers carry WB finishes in pints (sometimes half-pints), quarts, gallons, and aerosol sprays, usually in gloss, semigloss, satin, and flat sheens. Expect coverage similar to traditional finishes, about 400 sq. ft. per gallon. Stir it before every use [Photo C]. Strain material to remove dried or clumped finish if the can has been partially used.
Sand and prepare the wood, as for other clear finishes. Some finishers suggest raising the grain by applying water to bare wood first and lightly sanding, but letting the first coat of finish raise any grain works just as well. Then, sand to 220 grit or finer and apply a second coat [Photos D and E]. Sand to 220 grit for subsequent coats.
Apply WB finish with a synthetic-bristle brush; natural bristles tend to absorb the water in the finish and become too limp to brush well. Many finishers prefer bristles made of Taklon, a polyester filament [Opening photo]. Foam brushes work surprisingly well for WB finishes, too, and can be thrown away after use.
Lay on the finish with long, wet brushstrokes along the wood grain, maintaining a wet edge. Minimize brushing: Let the finish sit on the surface rather than try to scrub it into the wood. If the surface looks rough or uneven right after application, leave it alone. The finish dries quickly once brushed on, so going back over it just makes it look worse. Brush marks disappear and the finish selflevels as it dries.
Sanding and applying another coat will repair any flaws that don’t go away with drying. Apply at least three coats to achieve a high-quality finish. (With reapplication time as little as two hours, you can easily put on three coats in a day.)
To apply WB finish over an existing finish of any type, clean and scuff-sand the surface first. A lightly sanded shellac seal coat enhances adhesion.
You can also apply WB finishes with a conventional or high-volume, low-pressure (HVLP) spray gun. Most products need no thinning for spraying, although extender (available from a paint dealer) may be required in hot or dry conditions. Check the can label or the manufacturer’s website for nozzle and pressure recommendations.
Clean brushes or spray equipment with soap and water [Photo F]. Rags used during finishing and cleanup pose no fire hazard and can be thrown in the trash—another advantage over oil finishes.
Storage and disposal
Store finish in its original can or a tightly lidded glass or plastic container marked to show its contents. Keep the product from freezing. If it does freeze, put some drops on a piece of glass or metal to see if the finish dries within an hour or so. If it doesn’t, discard the finish. Discard finish that’s stringy, gummy, or smells sour.
Dispose of empty, dried cans in household trash. If a small amount remains in a can, leave the lid off to let the contents dry; or spread the leftover finish on newspaper or cardboard and let it dry. Check with local waste disposal authorities about disposing of larger undried quantities.
Waterborne finish can impart several looks, shown on beechwood. The unfinished wood at far left and the finished panel next to it (three coats of satin sheen applied with a foam brush) appear similar in color. The third panel shows three coats over super-blonde shellac. At the far right, three coats over dark-cherry water-based stain.
Cleanup instructions on the can label quickly identify WB finishes. Soap and water cleanup indicates a WB finish; mineral spirits, an oil-based product.
Sand between coats to give the surface tooth for the next coat. Sandpaper, sanding sponges, and nylon pads work fine, but do not use steel wool with waterborne finish.
Always stir WB finish; shaking creates bubbles that take a long time to dissipate. Milky-white appearance is normal in the can; the material dries clear on the wood.
Dish detergent Clean brushes with dish detergent and water. Work the suds into the bristles, rinse the brush thoroughly with running water, and pat it dry with paper towels.
Thoroughly clean off sanding dust to ensure a smooth finish. Rely on a tack cloth or damp rag to pick up the particles rather than just spread them around.