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.
Is Oil-Based or Water-Based Polyurethane Better?
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. Oil-based finishes also tend to “yellow” more so than water based finishes. This can be a good thing if you desire a “warmer” look to your finish.
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 Polyurethane
Stir and 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. Then you have to figure out a way to fix that!
Bubbles in polyurethane finish. Any way to fix? from r/woodworking
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
Brush and apply finish with the grain of the wood, using a sufficient, but not overly thick coat of polyurethane. Avoid overbrushing, but be sure use long strokes to brush out as many bubbles as possible. The few remaining bubbles will typically disappear within moments.
Repeat these steps until the desired level of protection is achieved. Two or 3 coats is normally sufficient. 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
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 maybe one or 2 more coats of the water-based polyurethane.
Additional Tips
When applying polyurethane on vertical surfaces, you may experience drips or runs. Be aware 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.
When applying polyurethane, always look at your surface from different angles. A bright light helps to pin-point areas that have been over-brushed or missed completely. Using this technique will help ensure consistent coverage.
