How to Apply an Oil-based Polyurethane Finish
While many do it yourselfers have switched to water-based polyurethane finishes for their furniture and woodworking projects, oil-based polyurethane has a more traditional look that brings out the wood’s natural beauty. These instructions will show you how to apply the polyurethane to achieve a high gloss durable finish. Common painting tools are all that are required as long as you have a quality natural-bristle brush, a well ventilated space, and some time.
Tools and Materials
China Bristle Brush or a synthetic bristle brush made for oil-based finishes
Sandpaper, 100, 150 and 220 grit
Wet/dry sandpaper, 400 grit
Clean, lint-free cloths
Automotive rubbing compound
Automotive polishing compound
If you want a smooth finish, you need to apply the polyurethane to a smooth surface. To achieve a smooth surface, sand the piece with progressively finer grits of sandpaper. Paper with a higher grit number removes the deeper scratches left by paper with a lower numbered grit. 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.
Look closely for any scratches left by coarse-grit sandpaper. You can wet the wood with a rag and some mineral spirits to look for fine scratches. Mineral spirits will darken the wood while it is wet, but will not raise the grain like water. Once you have reached the desired surface, remove the dust with a shop vacuum and a soft brush attachment. Continue removing dust by wiping down the wood with a lint-free cloth moistened with mineral spirits. Finally, to be completely sure the surface is free of dust, wipe it with a tack cloth. You can buy tack cloth or make your own by taking a lint-free cloth and moistening it with mineral spirits and a spoonful of polyurethane. Work the polyurethane into the cloth until it is even distributed. (If you plan to use a water-based urethane finish, the tack cloth should be made with denatured alcohol only.)
If you are working on a piece of furniture, you should apply the polyurethane in a clean room. If you cannot use an interior room and you need to work in your shop or garage, thoroughly clean the room of sawdust before you start. Ventilation is very important when working with solvent-based finishes, so work with open windows and possibly install a fan in the window. If you live in a dusty area, or there is pollen in the air, consider installing an air conditioning filter in the windows while you are applying the finish to provide air flow, but keep out insects, dust, and pollen.
If you are sensitive to fumes or you can’t achieve good ventilation, wear a respirator with an organic cartridge. Never place a fan placed near the piece you are working on as it will just blow dust onto your project.
1. Sanding Sealer
If you plan to stain the work surface, be sure to follow the directions on the can. Water-based stains will raise the grain of the wood so you will need to sand the surface again 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 the next step.
Use a sanding sealer to lock the loose fibers of the wood into place. You can buy sanding sealer or make your own using 2 parts polyurethane and 1 part mineral spirits. Brush the sealer on with a natural-bristle brush using long, even strokes. Remember to mix the polyurethane first before combining it with the mineral spirits in a clean container or jar.
Tips: Never shake a can of polyurethane. Shaking creates air bubbles into the mixture that’ll end up as bumps on the finished surface.
Dip your brush into the mixture about 1 in. 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.
2. Applying Polyurethane Finish
Within 24 hours of applying the sanding sealer, 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. Be careful not to use too much or you will get runs and drips. As soon as the surface is coated, brush over it again with the grain, from end to end.
Let the second coat dry for 24 hours, then apply the third and final coat.
If you want a glossy smooth finish, follow the instructions below discussing wet sanding.
3. Sanding and Smoothing
After the first coat of polyurethane you may notice a few specks of dust on the surface. Once the finish is completely dry to the touch, usually about 12 hours, remove any dried drips with a razor blade scraper. Be careful not to cut below the surrounding surface. As long as you’re careful small blemishes will disappear after you wet-sand the finish.
If you want a high-gloss hard finish, wet-sand the finish after each full-strength application has dried. This will remove any dust bumps or blemishes. Remember that each coat is fairly thin so be careful around corners and edges.
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. Use enough water to lubricate the sandpaper so you don’t rub through the finish. Sand just deep enough to remove the blemish.
Tip: Many experienced woodworkers recommend using a mixture of water and liquid soap to help lubricate the wet/dry sandpaper. Avoid using any type of oil as this will present problems when applying additional coats of polyurethane. Also the oil tends to conceal any imperfections in the finish.
When the surface feels smooth, wipe it with a moist cloth and then dry it with a dust-free cloth.
4. Final Coat
Brush on the final coat within 48 hours of the previous coat. Make sure that your room is as dust-free as possible and that your brush is clean. Even with this, you may need to repair some runs or sand out bubbles or dust.
If after the final coat you still need to wet-sand, you will need to polish the surface afterwards. If polishing is required, it’s best to wait at least 48 hours, or 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.
For even greater luster you can apply a polishing compound after the rubbing compound. The polishing 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.