When finishing a kitchen table, you want it to be beautiful but you also need it to be tough. If you exclude professional finishes such as catalyzed lacquer or urethane, polyurethane is probably the toughest finish you can find. Polyurethane is commonly sold in both water-based and oil-based formulas, and each has its pros and cons. By far the best way to apply polyurethane to a table is to spray it, but if you're sensitive to VOCs and are committed to brushing, it's best to use a foam brush to minimize streaking.
No Matter What, You Need to Sand
Whether you're spraying or brushing, the key to a professional finish is preparation. Sand the table top with a progression of sandpaper grits until the surface is smooth and scratch-free. If you have to make repairs with wood filler, do this before sanding to avoid having to come back and sand again, which can produce subtle but noticeable blemishes.
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For the best results, you'll need at least three coats of finish, but you may want to apply even more. Allow each coat to dry as directed (this may range from one hour to several hours), then scuff with #220-grit sandpaper before applying another coat. This is the technique professional finishers use to get the flawless finishes on expensive cabinetry. Just remember to wipe off all the dust with a damp cloth after each sanding. Wait for at least 24 hours after the final coat before using the table.
How to Spray Polyurethane Finish
When spraying, note the width of the spray pattern and make sure you overlap by at least half the width on each pass. After you've applied a coat, the table should look uniformly wet and glossy. If you see cloudy areas, it's fine to go back and give these an extra shot. Spraying offers a mirror finish, eliminates the possibility of drips and bubble formation and saves on time. The biggest negative of spraying your finish is obtaining, setting up, and cleaning the spray equipment. You can rent a professional-grade sprayer or purchase an inexpensive handheld airless sprayer. Be sure to ventilate the work area and always wear a respirator rated for solvent vapor.
How to Brush Polyurethane Finish
Brushing allows you to apply thicker coats than spraying does, and you can still get beautiful results if you follow a few guidelines. Use a foam brush to apply a smooth coat free of brush strokes. If you have to use a bristle brush, be sure to use one with synthetic bristles for water-based products. A natural-bristle brush goes limp in water, and you'll lose control. Be sure to always brush with the grain, as any stroke marks the brush leaves will be less noticeable if they run with the grain instead of against it.
If you choose to brush as opposed to spray, you'll need to take your time. Polyurethane products, especially water-based ones, tend to foam when you agitate them. The foam forms bubbles on the surface, and if they harden, they can be difficult to camouflage. Let the polyurethane sit for about 10 minutes after stirring to allow the foam to settle. Dip your brush gently and take your time painting the surface. If you really want that mirror finish, spray the final coat. This will create the impression that you sprayed all the coats and give the finish a flawless appearance.