Water-based finishes have improved quite a bit over the past few years – to the point where they make an excellent finish for just about every woodworker. In particular, they offer a lot of advantages for DIYers and hobbyist woodworkers, especially those working in small shops. You can use a water-borne finish in place of just about any other film finish (varnish, polyurethane, lacquer) on just about any wood surface (furniture, cabinetry, trim work, and flooring). While it can be sprayed on, it's likely that most DIYers and hobbyists will brush it on, which is what I do.
What is Water-Based?
If you've been woodworking for any length of time you've no doubt heard about water-based finishes, even if you haven't tried them. Essentially, it's any finish that uses water as a thinner and some kind of organic compound, such as glycol ether, as a solvent. If the product says to 'clean up with water' then it's a water-based finish. Water-borne is just another name for water-based – though perhaps a more accurate moniker.
Water-based finishes also contain resins (along with some chemicals) that are dispersed in the water, so you'll see these products variously labeled as water-based varnish or water-based polyurethane (containing a urethane or urethane/acrylic resin blend) or water-based lacquer (containing acrylic resin). The water and solvent evaporate, and the resin coalesces (merges) into a film on the surface of the wood, which is why they're referred to as coalescing finishes. As with any other film finish they're available in gloss, semi-gloss and matte sheens.
While there have been various problems associated with these finishes in the past, the new generation of water-based finishes are pretty darn good. Manufacturers are constantly improving both the quality of the resins they use, and their formulations (the secret recipes) for blending the various ingredients, to make the finishes easier to apply, tougher, more durable, and more resistant to scratching, heat, and solvents.
Advantages of Water-Based
There would be no reason to switch to a water-based finish if it didn't offer some tangible advantages. The most important reason, in my mind, is that they offer a level of toughness and durability comparable to other film finishes. They are also a lot quicker drying so that less dust will settle and dry on the finish. You can usually re-coat your project within a couple of hours.
Water-based finishes are also the clearest finishes you can get, and they won't yellow over time, as is common with oil-based finishes. However, some water-based finishes, such as General Finishes Enduro-Var) now have an amber cast that mimics the traditional oil-finished look.
They are also better for your health and the environment, and are safer to use. They have very low VOC (Volatile Organic Compound) levels, they're non-toxic, non-polluting, virtually odour free, and they're non-flammable.
Cleaning up after applying a water-based finish is a piece of cake. Water, a bit of soap, and you're done.
Some Issues With Water-Based
A common complaint with water-based finishes is that they raise the grain – which shouldn't be surprising given that there is water in the finish. While you could moisten the wood surface to raise the grain and then sand away nibs before applying the finish, it's quicker to use the first coat of the finish as a sealer. It's unlikely that anyone's shop is so dust free as to preclude denibbing between applications of finish anyway. I use P320 or P400 paper between coats.
Water-based finishes are more finicky when it comes to temperature and humidity. They like to be applied when the temperature is between 70 and 80-degrees, and the humidity is no more than 70%. If your shop is too cool, turn up the heat. You can even warm up the finish by putting the can in a bucket of warm water. If your shop is too warm or too dry, add a dry-time extender or distilled water to thin the finish. You'll find this also helps when finishing a large project. Make certain you use an extender specifically designed to work with water-based finishes. The only one I've used is General Finishes Enduro Extender.
The characteristics that make water-based finishes exceptionally clear, which is often desirable on light coloured woods, is not so attractive on darker woods, which can tend to look drab. To enhance the grain you can stain or dye the wood before applying the finish, or add a dye directly to the finish.
Apply It With Brush
Because I now work in a fairly small shop I apply finishes with a brush. I've found that I get much better results on water-based finishes using a fine synthetic bristle brush with flagged bristles (Chinex or Taklon are good choices, along with Gramercy synthetic badger-hair brushes). I've not used foam brushes or applicator pads so can't attest to their efficacy.
You'll need to modify you're technique a tad. Because these finishes don't flow as well as other film finishes, and dry so quickly, you don't want to go back over your work. Lay down a single coat, and then walk away. Don't worry if you miss a spot – you'll cover it on your next application. The finish will be dry in a couple of hours, enabling you to lay down a follow-up coat. Because of their high solids content water-based finishes build up quite fast – typically three or four coats is all it takes.
It's a good idea to use your brush solely with water-based finishes, and to clean it thoroughly after each use. Work the bristles back and forth under warm water to remove the excess finish, and then add a little dish soap to the brush and work it through completely. End off by rinsing the bristles thoroughly, and wrap the brush in Kraft paper or Scott towels held in place with an elastic band. This allows the bristles to dry while maintaining their shape, and keeps shop dust from contaminating the bristles (and your finish).
Once a can has been opened, I always strain the finish through a paper paint strainer before I use it again. Any dried bits of finish that fall into the can won't re-dissolve – they'll just end up on your project.
Just like other film finishes you can rub out a water-based finish. It's best to wait at least a couple of weeks, longer if you can. I use the same technique as when rubbing an oil-based finish, except that I use synthetic pads rather than steel wool and substitute mineral oil for mineral spirits. I've had better luck rubbing out water-based lacquer finishes than water-based polyurethane. Possibly because there's more acrylic in the lacquer finish.
If you've not used a water-based finish before, or it's been quite some time since you last used one, give it a try. I think you'll be quite pleased with the results.