How to apply polyurethane?

1: Seal the surface

Thin your oil-based polyurethane with mineral spirits. Brush the sealer on with a natural-bristle brush using long, even strokes. Be sure to catch any runs.

If you plan to stain the work surface, be sure to follow the directions on the can. Whether the stain you use is oil-based or water-based, be sure the surface is clean and dry before you brush on the seal coat.

Water-based stains will need a fine sanding with 220-grit sandpaper after the stain is dry. Also, some stains are self-sealing. Check the can for this information. If the stain is self-sealing, go to Step 2. There’s no need to duplicate the process.

To seal the wood before top coating, thin your oil-based polyurethane to one part mineral spirits and two parts polyurethane (first, gently stir the contents in the can). Pour the products into a glass jar and gently stir the mixture with a flat stir stick.

Never shake a can of polyurethane! You’ll introduce air bubbles into the mixture that’ll end up as bumps on the finished surface.

Use a good-quality natural bristle brush to apply oil-based polyurethane. Exploded-tip synthetic brushes (each strand is divided into multiple strands at the tip) may introduce air bubbles into the finish.

Load your brush by dipping it about 1 in. into the mixture and brush the wood with long, even strokes from end to end (Photo 2). Keep a wet edge by overlapping each pass until the surface is completely coated. Catch any drips with your brush and smooth them into the surface. Before I clean and put the brush away, I like to wait for five minutes then look for any further dripping.

2 Apply two undiluted coats of polyurethane

Brush on the first polyurethane coat

Use undiluted polyurethane varnish straight from the can for the second and third coats. After you've wet the entire surface, go back and overlap each stroke across the entire surface, following the direction of the grain.

Within 24 hours of applying your seal coat, brush on a coat of finish right from the can. Avoid wiping the brush on the rim of the can because this may introduce air bubbles. Spread the varnish over the entire surface with long, even strokes. Don’t use too much or you’re likely to get runs; use just enough to get a nice, even coat without dry spots. As soon as the surface is coated, brush over it again with the grain, from end to end (Photo 2).

Overlap your strokes to get a uniform coating; now’s also the time to catch any drips, especially along the bottom edges.

Again, wait five minutes after the first coat and inspect for any drips. Let this coat dry for 24 hours, then apply the final coat.

3: Fix the nasties

Shave off bumps

Cut away any dried drips with a razor blade. Don't cut below the surrounding surface. Small blemishes will disappear after you wet-sand the finish.

After the first full-strength coat, you might find a run or two and maybe a gnat or dust speck that settled on the surface before it dried. Once the surface is fully dry to the touch (at least 12 hours), cut any drips away with a razor blade as shown in Photo 3.

Be careful not to cut completely through the finish. Just cut deep enough to blend with the surrounding surface. It’ll require the delicacy of a surgeon; take your time. After the surface has dried for 24 hours, any small blemishes can be removed by wet-sanding with 400-grit sandpaper mounted to a sanding block. Wet-sand the surface by dipping the sandpaper in water and using circular strokes to remove the blemishes and any dust bumps (Photo 4). Use enough water to lubricate the sandpaper so you won’t burn through the delicate finish. Sand just deep enough to remove the blemish.

Too many strokes will dig right through the finish to the wood below and create a finisher’s nightmare. (You’ll have to fuss with spot finishing and blending.) When the surface feels smooth, wipe it with a moist cloth and then dry it with a dust-free cloth.

4: Apply the final coat

Polish the surface

Polish the surface only if you had to sand the final coat to remove blemishes. Wait at least 48 hours, then use an automotive rubbing compound. For greater luster, follow that with a polishing compound.

Brush on the final coat within 48 hours of the previous coat. Use the same care with this coat. With any luck, you’ll have a smooth, bump-free finish after it dries and you’ll be done! If luck has passed you by, you may need to repair some runs or sand away bubbles or dust.

Again, use the same methods as before, but keep in mind that if you wet-sand at this stage, you’ll need to polish the surface. If polishing is required, it’s best to wait several days for the finish to cure. Dampen a clean cotton cloth with water and apply automotive rubbing compound to the surface in a circular motion.

The compound has a fine abrasive that’ll remove any scratches left by the 400-grit sandpaper and restore the luster in the finish. After rubbing, let the surface dry and buff the finish with a clean, dry cloth. If the finish is still a bit cloudy, apply more automotive polishing compound using the same method. Let it dry, then buff the surface. You’ll get a great-looking shine that’s worthy of a pro.