Getting a smooth, blemish-free finish with oil-based polyurethane is within your grasp if you follow the steps in this article. Oil-based polyurethane varnish brings out wood's natural beauty and grain. Our 8-step approach shows you how to apply the varnish successfully. A good-quality natural-bristle brush, a reasonably dust-free, well-ventilated space and some patience are all you need.
Automotive polishing compound
Automotive rubbing compound
Finish your woodworking project or furniture refinishing job with an oil-based polyurethane varnish to bring out the wood’s natural beauty and grain. Learn how to prep the wood surface and successfully apply a flawless finish with these detailed steps.
We use an oil-based polyurethane finish in this story, though you can also use water-based urethane finishes with great results. But the color and depth you get are no match for oil-based urethanes. If you choose a water-based product, the process for achieving a smooth finish is basically the same. Just be sure you use a tack cloth to clean off dust that’s appropriate for water-based finishes, like a cheesecloth moistened with denatured alcohol.
Have a lot of polyurethane to apply? Watch this video for the fastest way to get it done:
Project step-by-step (8)
Sand the Surface
Sand your project with progressively finer grits of sandpaper.
Note: Paper with a higher grit number removes the deeper scratches left by paper with a lower-numbered grit.
Pro tip: Most projects need an initial sanding with medium sandpaper (100-grit) followed by a fine paper (150-grit) and then an extra-fine sanding with 220-grit.
Have an exhaust fan facing out through a window to remove vapors and crack a window at the other end of the room to let fresh air in. If you’re sensitive to fumes or can’t get good ventilation, it’s a good idea to wear a respirator with an organic cartridge. A fan placed near the workpiece is bad news and will only blow dust right on your project. The idea here is a gentle flow of air in and then out.
Remove the Dust
Once the wood is blemish-free, remove the dust.
Pro tip: Use a shop vacuum with a soft brush attachment, followed by a wipe-down with a clean, lint-free cloth moistened with mineral spirits.
As a last dusting step, wipe the surface down with a tack cloth.
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Do It Right, Do It Yourself!
Seal the Surface
Thin your oil-based polyurethane with two parts polyurethane and one part mineral spirits.
Pro tip: Pour the products into a glass jar and gently stir the mixture with a flat stir stick.
Brush the sealer on with a natural-bristle brush using long, even strokes. Be sure to catch any runs.
Note: Some stains are self-sealing. Check the can for this information. If the stain is self-sealing, you can skip this step.
Pro tip: Never shake a can of polyurethane! You’ll introduce air bubbles into the mixture that’ll end up as bumps on the finished surface.
Pro tip: Use a good quality natural bristle brush to apply oil-based polyurethane. Exploded-tip synthetic brushes (with each strand 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.
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.
Apply the First Coat of Polyurethane
Within 24 hours of applying the seal coat, brush on a coat of finish right from the can.
Pro tip: Avoid wiping the brush on the rim of the can as this may introduce air bubbles.
Spread the varnish over the entire surface with long, even strokes.
Pro tip: 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.
Overlap your strokes to get a uniform coating.
Catch any drips, especially along the bottom edges.
Shave Off the Bumps
Once the surface is fully dry to the touch (at least 12 hours), cut any drips away with a razor blade.
Pro tip: Be sure not to cut below the surrounding surface.
Wet Sand the First Coat
After the first coat has dried for 24 hours, remove small blemishes by wet-sanding with 400-grit sandpaper mounted to a sanding block.
Dip the sandpaper in water and use circular strokes to remove the blemishes and any dust bumps.
Pro tip: Use enough water to lubricate the sandpaper so you won’t burn through the delicate finish. Sand just deep enough to remove the blemish.
When the surface feels smooth, wipe it with a moist cloth and then dry it with a clean, dust-free cloth.
Apply the Second Coat
24-48 hours after you apply the first coat, apply the second and final coat using the same procedure as the first.
Repeat Steps 5 and 6 if Needed
Once the second coat is dry, shave off any bumps (per Step 5) and wet sand the surface (per Step 6).
Note: Only follow these steps if absolutely necessary. If you have to wet sand the second coat, you will need to polish the surface.
Polish the Surface if You Wet-Sanded the Second Coat
Wait at least 48 hours, then polish the finish using an automotive rubbing compound.
Dampen a clean, cotton cloth with water and apply automotive rubbing compound to the surface in a circular motion.
Note: The compound has a fine abrasive that’ll remove any scratches left by the 400-grit sandpaper and restore the luster in the polyurethane finish.
After rubbing, let the surface dry and buff the finish with a clean, dry cloth.
Pro tip: If the finish is still a bit cloudy, apply more automotive polishing compound using the same method. Let it dry, then buff the surface.
Pro tip: For even greater luster, apply a polishing compound as the final step.