Polyurethane is widely revered as one of the most durable yet easy-to-apply protective wood finishes. Polyurethanes are now available in both oil-based and water-based, and there are differences in the way in which both are applied and in durability. However, for many projects that will see a lot of wear and tear, few finishes are as appropriate as applying polyurethane for the final touch.
Which is Better? Oil-Based or Water-Based Polyurethane?
The decision to use an oil-based or water-based polyurethane will depend on your project. Oil-based polyurethanes are easier to apply and less temperamental than water-based polyurethanes, as two or three applications will usually be enough to protect your project. Oil-based polyurethane finishes are susceptible to brush marks, and they take some time to dry, which may subject your finish to bugs or dust.
Water-based polyurethane versions dry much more quickly, are a bit more self-leveling and have less odor when applying than the oil-based versions. However, the water-based polyurethanes tend to raise the grain of the wood, are susceptible to water marks and can be somewhat temperamental when using with stains. Water-based polyurethanes tend to have a milky whitish look when applying, but this should disappear as the finish dries.
Working with Polyurethanes
First of all, stir, but never shake a can of polyurethane.
Why? Shaking a can of polyurethane will introduce numerous bubbles into the product that will show up in your final finish. Instead, stir the product gently but thoroughly before each use.
With either type, always work in a clean but well-ventilated area. You may choose to thin oil-based polyurethanes with mineral spirits or naphtha, but for most applications, this will not be necessary.
Applying an Oil-Based Polyurethane
One can apply an oil-based polyurethane using a fine-bristled brush, a clean cloth or a foam brush. Avoid using inexpensive bristle brushes, as these will tend to leave obvious brush strokes. I prefer to use foam brushes, as they are inexpensive and provide an even finish without many brush strokes. Brush with the grain of the wood, using a sufficient, but not overly thick coat of polyurethane. Avoid over brushing, but be sure use long strokes to brush out as many bubbles as possible. The few remaining bubbles will typically disappear within moments.
After the first coat has dried (I typically wait 24 hours), lightly sand the entire surface (again, with the grain) with 320-grit sandpaper. The polyurethane will sand easily, so be careful not to sand through the thin coat and damage the stain underneath. Wipe off all dust caused by the sanding before applying the second coat.
Repeat these steps until the desired level of protection is achieved (two to three coats is usually enough).
After your final coat, you may choose to rub out the finish with #0000 steel wool to a consistent sheen level, followed by an application of paste wax for a nice luster.
Applying a Water-Based Polyurethane:
Water-based polyurethanes don't match well with oil-based stains, so if you're applying over stain you'll want to "rough up" the stained surface slightly before applying your water-based polyurethane using some synthetic steel wool. Since oil and water don't mix, this will help the polyurethane to avoid beading on the surface like water on a freshly-waxed car.
Apply a very thin coat of polyurethane with a fine brush, foam pad or cloth. Work with the grain, and avoid applying too much polyurethane to avoid raising the grain.
The initial coat should be dry within a couple of hours, and a second coat can be applied. If applying in this manner, one shouldn't need to sand between coats as with the oil-based version. However, in order to get the same amount of protection, you may need to add a dozen or more coats of the water-based polyurethane.
When applying polyurethane on vertical surfaces, you may experience drips or runs. Be cognizant of this as you apply your polyurethane, using thinner coats on these surfaces. Should a run occur, you may be able to sand out the run or carefully remove it with a sharp razor blade (followed by a sanding to feather in the blemish).
When applying polyurethane, always look at your surface from different angles. I like to work with a bright light nearby, and by standing and looking at the reflective surface of the fresh polyurethane from different angles on the opposite side of the project, I can see areas that I might have over-brushed or missed completely. Using this technique will help ensure consistent coverage.