Working with WaterbornesComments (0)
This article is from Issue 19 of Woodcraft Magazine.
Waterborne or “water-based” finishes have been around for almost 20 years, but many woodworkers have not yet cracked open a can. Part of the problem was the bad rap earned by a few first-generation formulas. These finishes were difficult to apply, offered little protection, and had a color quality that sucked the life out of even the best-looking boards. Lucky for us, times have changed. Thanks to major formula improvements, more woodworkers are beginning to appreciate why these cans were put on the shelves in the first place.
Shifting from your old favorite finish to waterborne is like transitioning from oil-based to latex paint. It isn’t hard, but there is a learning curve. With the help of Ben Svec, a 10-year water-base veteran (and the designer and builder of our cover project), we’ve made a simple five-step sequence you can use to ensure that you’ll get a successful finish right from the start. With Ben’s help, we’ve compiled additional information and some problem-preventing tips you can use to seamlessly blend this new finish in with the tools and techniques you already use in your shop.
Why Go Waterborne?
Water-based finishes aren’t that different from the other film-forming finishes you’re used to working with. Like those, waterbornes are made up of urethanes (for toughness) and acrylic (for flexibility). The big difference is that in this case, water—not some potentially toxic solvent—carries those resins onto the wood. For Ben, having H2O work as the carrier instead of a volatile chemical has three big benefits:
Water-based finishes are non-flammable. This fact gives Ben the freedom to use his HVLP sprayer—without an explosion-proof spray booth. A low-VOC (low-fume) finish is just as beneficial for woodworkers in small basement and garage shops hoping to avoid filling their work spaces and homes with solvent odors. When brushing on a few coats, “adequate ventilation” just means cracking open a nearby window.
When applied with a sprayer, waterbornes skin over in five minutes (it’s slightly slower with a brush). This is almost as fast as a lacquer. A super-fast drying time not only prevents settling dust from sticking to and ruining a smooth coat, it also gives Ben the ability to start a finishing project on a Saturday and ship it out by Monday or Tuesday.
Oil-based and lacquer finishes require equally toxic solvents for cleaning up brushes, sprayers, and spills. With waterbornes, it’s a simple soap-and-water routine. (Make sure that you clean your tools before the finish cures; if you don’t, you’re out of luck.)
4 STEPS FOR SUCCESS
Using Ben’s step-by-step sequence even first-time waterborners can get a great-looking finish right from the start. Read each step, then check the applicable sections for additional info and problem-preventing tips.
A careful sanding up to 180-grit is fine enough. Film-forming finishes don’t need a super-smooth surface.
Even though water-based finishes have a reputation for grain-raising, Ben found that he can usually skip the water wipe-down and de-whiskering. Instead, he simply applies two coats of General Finishes High Performance (HP) Polyurethane Gloss to seal the bare wood and establish a coat that’s thick enough to sand.
Note: If you use a water-based stain (see “Color Control”), you’ll need to add a step. Pretreat the bare wood with distilled water, allow it to dry, then lightly sand off any wood whiskers before staining. If you wait to do all your sanding after you’ve stained your work, you may rub off most of the intended color.
2 Seal & smooth
Using a sprayer, Ben applies two light coats of HP gloss (you can also use a compatible water-based sealer such as EF Sanding Seal, Woodcraft #813684) to lock down the grain and protect any color that was added in Step 1. Pay close attention to drying windows. For a few hours, partially-cured finish will bond with the fresh coat. When that time’s up, you’ll need to wait for the finish to completely cure, then lightly sand the surface to guarantee a good bond.
Ben gives the second coat about four hours to cure before knocking off the nibs or stray wood whiskers with 320-grit sandpaper or fine-grit (gray) synthetic abrasive pad. (See “What To Watch For,” on page 33 for additional tips to help you avoid unwanted staining or adhesion problems while applying the sealer or final coats.)
3 2-Coat top-off
Water-based finishes are relatively thin, especially when sprayed. Ben applies two more coats to ensure that his work is well covered. Ben prefers satin not only for its more natural-looking shine but also because it conceals minor imperfections that would jump out under a coat of gloss finish.
4 Final run-out
Use an ultra-fine synthetic pad to smooth out any bubbles, dust bits, or brush marks that might be trapped in the finish. When your finish looks (and feels) smooth, buff on a light coat of wax.
Until now, most “clear” finishes contained some amber-hued component that would help “warm up” the look of the wood. Waterbornes are different. General Finishes HP has a slight amber tint, but some water-based finishes are almost crystal clear. “No added color” can be advantageous when finishing maple or pine, but when finishing oak, walnut, or cherry, you may want to add a little extra warmth to your project. Here, you have three different choices to choose from:
Oil-based stains are compatible with water-based finishes, provided that the stain is completely dry. (If it’s not, the finish might not stick.) Give the stain at least two days to dry, or seal it under a coat of dewaxed shellac, before applying any water-based topcoats.
Water-based stains dry faster than oils, and they won’t create compatibility problems, but because the water will raise the grain, you will need to budget additional sanding time. Before staining, spray the project with distilled water or rub it down with a damp cloth. Allow it to dry, then knock off any wood whiskers by lightly sanding with 320 grit.
If you’re applying your water-based finish with a brush, you’ll need to be careful not to redissolve the stain. Pulling even a little stain from the wood can create a muddy-looking finish. To prevent this, seal the stain under a light coat of dewaxed shellac.
Adding a few drops of a concentrated, water-soluble dye, such as Honey Amber TransTint (Woodcraft #128481), combines staining and sealing into a single step. Note that doing this turns your clear finish into a toner—each additional coat will make the wood a little darker. Although quick and effective, finding the perfect shade may require adjustment and more experimentation.
What To Watch For
No finish is completely foolproof; waterbornes are no exception. Here are a few things you should watch out for before and during the finishing process.
Waterbornes don’t play well with other chemicals. Silicones, oils, or even the trace amount of solvent in a spray gun after cleaning, can cause adhesion problems. (To be super-safe, Ben keeps one spray gun reserved exclusively for waterbornes.)
The more contaminates you can keep out of the picture, the better. Use distilled water when raising the grain or thinning a water-based finish. When sanding between coats, use synthetic abrasive pads or open coat paper. Stearated sandpaper and steel wool can leave residue that may affect adhesion or even create rust-spots under the next coat. Because even the chemicals used in tack cloths can create problems, Ben uses a brush and vacuum to pick up sanding dust and loose grit.
The ideal finishing window is between 60° and 80°F. Above that range, waterbornes dry too quickly; below that, they may not cure at all. High humidity can also slow down the cure time.
Better Brushwork Without Bubbles
Spraying is fast, but you can get great results with a brush. Synthetic bristles and short-napped pads both work well, but Ben prefers inexpensive foam brushes because he can toss them when he’s done. The trick is to work quickly and smoothly so that the finish has plenty of time to level itself out and bubbles are kept to a minimum.
To load the brush, dip the foam about halfway into the container. As you remove it from the finish, give it a few seconds to drip off any excess, then lay it on your work and drag it across the surface. Keep your wrist action to a minimum, especially at the end of the stroke. Focus on pulling a line of finish across the wood.
It’s OK to tip-off each pass right after brushing it on, but resist the temptation to smooth out earlier passes. Most of the bubbles and brush marks will level themselves out. Those that don’t can be sanded down later. Correcting “bad” brushwork almost always winds up doing more harm than good.
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