Tips for Sanding Polyurethane (How to Get a Smooth Finish)

Published on: June 10, 2022
Written by Eric Devin / Fact-checked by David Rowan

Polyurethane is a polymeric substance that comes in a variety of shapes and sizes. It may be either stiff or flexible, and it is the material of choice for a wide range of end-user applications, including refrigerator and freezer insulation. insulating a structure

Polyurethane is one of the most common materials used in home furnishings such as furniture, bedding, and carpet underlay and is most commonly seen in the form of flexible foam. Flexible polyurethane foam serves as a cushioning material for upholstered furniture, making it more durable, pleasant, and supportive.

Thermoset polyurethanes are not regarded as “plastic” in the strictest sense. The long flexible polymer molecules that make thermoset polyurethanes, on the other hand, turn this material into a real elastomer.

Polyurethane performs better at high and low temperatures than plastic, is more flexible, and does not fracture under shock or other mechanical stress.

What Is the Best Grit Sandpaper for Polyurethane?

It’s best to use the finest grit sandpaper you can get. Many commercially made sanding sponges with fine-grit sanding surfaces can be cleaned in water when the sponge’s surface becomes clogged with whatever you’re sanding — wood, paper, polyurethane finish, etc.

You’re more likely to leave obvious scratches and grooves on the surface of your varnished object if you use very coarse grain sandpaper, and you’ll wind up removing more of the initial coat of polyurethane than you desire.

How Do I Get a Smooth Finish With Polyurethane?

Want a wood finish that’s as smooth as glass? Without the use of professional tools or spray equipment, you may obtain a shiny, smooth surface. Simply follow these simple instructions.

First

Using 180 or 220 grit sandpaper, sand the wood surface. Wet sanding is preferred over dry sanding because the wet grit adheres to the surface better.

Second

Go for a thin coat of sealer to all wood surfaces, covering every square inch, and let it cure for approximately an hour before applying a second coat (or more).

Third

Apply two coats of polyurethane to all wood surfaces with a paint roller or brush. Allow no time for the layers to dry between them. You’ll get better results if you do it in warm, humid conditions since the polyurethane dries faster, resulting in a smoother polyurethane varnish than if you apply it in colder temps and/or lower humidity levels.

A pump-up sprayer can also be used, although it’s not as excellent at smoothing out bubbles and leveling up uneven finish spots. If you need to touch up after your initial polyurethane coat, make a 16:32 solution of rubbing alcohol and water and pour it into a new polyurethane bottle before spraying again.

Fourth

Put 150 grit wet paper over the top layer as the last step to create a really smooth finish. Allow a minimum of 24 hours for everything to cure in warm, humid settings so moisture may fully escape from the wood surface before doing anything else, such as furniture, flooring, or other items where humidity is undesirable.

FAQs About Sanding of Polyurethane

What Is the Best Method to Apply a Second Coat of Polyurethane?

Between applications of polyurethane, sanding

• Using a high-quality natural or foam brush, apply a THIN layer of Minwax® Fast-Drying Polyurethane.
• Allow 4-6 hours for drying. Then, using fine sandpaper (220 grit), softly sand the whole surface to provide an equal finish and perfect adherence.
• Apply a second layer of paint. If you want a third coat, repeat step 5 before applying it.

Should You Sand Between Coats of Polyurethane?

If you don’t sand between applications of polyurethane, nothing happens with some types of polyurethane. However, if you avoid sanding dust nibs and brush marks on the finish, most polyurethane will not adhere effectively. The polyurethane coating will peel off or distort with time.

It takes a long time to sand thoroughly between coats of polyurethane. Getting the wooden surfaces ready is, without a doubt, the most difficult step. Even after applying the very first coat of polyurethane, it takes some time.

Worse, unless you’re an expert, you’ll almost certainly have to utilize hand sanding methods instead of machines, which will add to your wait time.

Manufacturers understand that you don’t have much time, and they want to assist you in moving on as soon as possible. That is why we now have a lot more fast-drying polyurethanes compared to when I first started woodworking many years ago.

The treatments that don’t require wet sanding between coats are the next step up from fast-drying polyurethane finishes; all you have to do is clean the wood floor with a tack cloth.

Because there aren’t many of them on the market, research is limited and good feedback is scarce.

Can I Sand a Polyurethane Finish?

No, the final layer of polyurethane does not need to be sanded. The only time you should sand the last layer of polyurethane is if the surface isn’t completely smooth. When applying polyurethane, a variety of issues might arise, including dust nibs, bubbles, and streaks.

In Polyurethane, How Do You Get Rid of Brushstrokes?

Using a paint stir stick, thin your polyurethane with paint thinner. Brush markings keep their form and do not settle out when polyurethane is too thick. The product is thinned to create a smoother final surface. To discover what works best for you, try experimenting with different quantities of paint thinner.

Is It Possible to Sand Between Layers of Polyurethane Using Steel Wool?

You may produce a consistent surface for the next coat of polyurethane by using steel wool or sandpaper, which looks nicer, more professional, and so on. Only the coarseness of the sandpaper or steel wool makes a difference in my experience, given the finish is dry.

How Do You Make Polyurethane Shiny After Sanding?

A polyurethane finish is a hard protective layer that absorbs scratches and dings that would otherwise damage a surface permanently. By polishing the polyurethane using mild abrasives, you may reverse the process and get a high-gloss finish that gives your surface a smooth, bright, and scratch-free appearance.

Only one thin coat is required. Then, using 1,500-grit sandpaper or a piece of a brown paper bag, remove any dust nibs. Finally, buff the finish to a high sheen using automotive paste wax and a soft cotton cloth or polishing pad. Fine abrasives are used in the shot below to polish the finish even further.

Final Words

Polyurethane is a versatile material that may be used on a variety of surfaces. They offer luster, attractiveness, and protection to the surface they’re applied on. However, to get the most out of your finish, make sure you employ the proper application procedures.

It’s possible to sand it for a smooth finish, but keep in mind the other options. Perhaps no sanding will work for you as well! In any case, I’m sure you’ve built a lovely wooden artwork that deserves to be appreciated. 

Best of luck with your project’s completion!