Polyurethane is widely revered as one of the most durable yet easy-to-apply protective wood finishes. Polyurethanes are commonly available in both oil-based and water-based formulas, and there are minor differences between the two in both performance and application. Standard polyurethane is applied with a brush, but there are also wipe-on formulas that are applied with a rag, as well as a spray finishes in aerosol cans. Regardless of the type you use, if your project will see a lot of wear and tear, few finishes are as appropriate as polyurethane for the protective top coats.
Which Is Better—Oil-Based or Water-Based Polyurethane?
The decision to use an oil-based or water-based polyurethane largely depends on your project and your preferences. Oil-based polyurethanes are somewhat easier to apply and can be less temperamental than water-based formulas. They're also a bit thicker and contain more solids, requiring two or three coats where water-based poly may need three or four. However, oil-based polyurethane finishes are susceptible to brush marks, and they take much longer to dry, which can slow down your project and possibly increase the risk of getting bugs or dust in your finish before it dries.
Water-based polyurethane versions dry much more quickly, are a bit more self-leveling, and have less odor when applying than oil-based versions. On the downside, water-based poly tends to raise the grain of the wood, is susceptible to watermarks, and can be temperamental when applied over some wood stains.
Color is another differentiator. Oil-based polyurethane typically adds a warm amber glow to wood, particularly to lighter wood species, such as white oak, maple, or birch. Water-based formulas generally are more neutral or clear. Water-based poly has a milky white look when it goes on but turns clear as it dries.
Tips for Working with Polyurethane
First of all, stir—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, just stir the product gently but thoroughly before each use.
Apply the finish in a clean, well-ventilated area. Polyurethane takes hours, not minutes, to dry; that's a lot of time for dust to settle or bugs to land on the surface, marring the final product. Both water-based and oil-based products give off strong fumes as they dry (although oil-based is decidedly worse), so proper ventilation is a must. Just don't finish your work outdoors, where you can't control dust, bugs, and other flying finish-ruiners.
It's best to apply polyurethane to flat (as in level) surfaces so the finish can self-level and is less likely to drip. When applying polyurethane on vertical surfaces, you may experience drips or runs. Minimize this problem by applying thinner coats or by switching from standard brush-on poly to a wipe-on or spray finish, both of which can be applied in very thin coats. If you end up with runs or drips, try to sand them out when sanding between coats, or carefully remove them with a sharp razor blade (followed by a sanding to feather in the blemish).
As you're finishing each fresh coat during application, check your work with a bright side-light. Crouch down so you see the light reflecting off of the surface. This highlights imperfections, such as bumps, bubbles, ugly brush marks, and spots that you simply missed or where the finish is too light. You can fix these problems when the finish is still wet but not once it starts to set up.
Prepping the Wood
As with all wood finishes, good results depend on smooth, clean wood surfaces, but this goes double with clear finishes such as polyurethane. Sand your wood with at least 220-grit sandpaper. For open-grain woods (like oak, ash, or walnut), you can apply a wood grain filler before the polyurethane, to create an ultra-smooth finished surface.
Clean the wood very thoroughly to remove sanding dust before each new coat of polyurethane, using a vacuum (if available) and a tack cloth. You can also use a rag moistened with mineral spirits (for an oil-based poly) or cheesecloth moistened with denatured alcohol (for a water-based poly).
Applying Oil-Based Polyurethane
You may choose to thin oil-based polyurethanes with mineral spirits or naphtha, but for most applications, this is not necessary; check the manufacturer's recommendations on the product label. Thinning can help the finish flow into fine details and nooks and crannies with less buildup.
Apply oil-based poly using a fine-bristled brush (natural or synthetic bristles) or a foam brush. Avoid inexpensive bristle brushes, as these tend to leave obvious brush strokes. Foam brushes are inexpensive (and disposable) and work well for most flat surfaces. Bristle brushes are better for molded edges and fine details.
Brush on the polyurethane so the brush strokes are parallel to the grain of the wood. Use a sufficient, but not overly thick, coat of finish. Complete each area with long, straight strokes to brush out as many bubbles as possible. The few remaining bubbles will typically disappear within moments.
After the first coat has fully dried (according to the manufacturer's directions), lightly sand the entire surface (again, parallel to the grain), using 320-grit sandpaper. The polyurethane will sand easily, so be careful not to sand through the thin coat and damage the stain or wood underneath. Remove all dust with a vacuum and tack cloth before applying the second coat.
Repeat these steps until the desired level of protection is achieved. For protection, two coats are the minimum, but floors and anything else that will see hard wear or occasional moisture should get at least three coats. Each coat also makes the finish a bit smoother. 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 prevent the polyurethane from beading on the surface, like water on a freshly waxed car.
The basic application technique for water-based poly is similar to that for oil-based. 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 you can then apply a second coat. If applying in this manner, you may not have to sand between coats as you do with oil-based poly; check the manufacturer's recommendations. Plan on at least three coats of water-based poly on lightly used projects and at least four coats on floors and any pieces that need maximum protection.
Wipe-On and Spray Polyurethanes
The primary advantage of wipe-on and spray polyurethanes is thinness. Both can be applied in ultra-thin coats that result in less buildup in nooks and crannies, provided you don't apply too much. Wipe-on poly is applied with a clean, lint-free rag. Spray poly goes on just like spray paint. Some woodworkers like to use spray poly for a thin top coat over base coats of conventional polyurethane, producing a final smooth coat without brush strokes.
Both wipe-on and spray formulas are a good solution for tricky applications, like fine details or vertical surfaces. The light application reduces drips and buildup. The primary drawback of these thinner formulas is that you likely will need more coats for a good, protective layer—perhaps five or six coats instead of two or three with conventional brush-on poly.