How To Apply Polyurethane Without Bubbles (8-Step Guide)

Applying polyurethane is a relatively easy process that can add years to the life of your flooring. The most common problem people have is preventing bubbles from forming within the layers of polyurethane. This can be avoided by using the correct supplies and taking your time when prepping and applying polyurethane to your floors.

What You Need To Know About Polyurethane

Polyurethane is a plastic polymer that is used in a wide variety of products. In its liquid form, it is commonly applied to wood as a protective layer from water damage and scratches. Depending on the finish of polyurethane you choose, it can also add a nice sheen to your wood that helps hide blemishes.

Properly applying poly will require applying a different number of coats and some extra supplies depending on which type and finish of polyurethane you choose.

Water-Based Vs. Oil-Based

Polyurethane comes in two different types, water-based and oil-based, and they both have their own pros and cons. Depending on your project and the amount of time you want to spend on it, one will always be a better candidate than the other.

Water-Based Polyurethane

Water-based polyurethanes dry clearer and quicker than oil-based polys. They also don’t give off as much of an odor, making them favorable for interior projects. 

On the other hand, they generally require you to apply more coats, and they don’t withstand heat or UV exposure as well as their oil-based counterparts. 

Water-based products are generally better for surfaces such as cabinets, window frames, railings, doors, and flooring in less-trafficked rooms.

Oil-Based Polyurethane

Oil-based polys are much more durable than water-based products, especially when it comes to heat and UV exposure. This makes it a better candidate for exterior flooring like deck boards. 

Conversely, they can change from a clear-yellow to an amber or light brown color over time, dry slower, and produce a much more pungent odor when wet than water-based polyurethanes.

Supplies You’ll Need For Applying Polyurethane

Applying polyurethane will require different types of applicators and supplies based on the size and shape of your project. Let’s take a look at all of the supplies you will need for your next project.

  • Very soft-bristled brushes or roller
  • Mineral spirits
  • Denatured alcohol
  • Orbital sander or sanding block
  • 80-120 grit sanding pads
  • 180-220 grit sanding pads
  • Stir stick
  • Cloth or sponge
  • Fan or respirator

How To Apply Polyurethane Without Bubbles (8 Steps)

Step 1: Select The Proper Applicator

Since we are applying polyurethane to flooring it is best to use a brush, roller, or pad. Though brushing will take longer, it is recommended to go this route over a roller. Rollers can create bubbles and an uneven layer if you aren’t consistent with the amount of pressure you apply.

If you choose to use a roller make sure that it is made from lambswool, microfiber, or foam. This is especially important for oil-based polys because they will absorb the urethane better.

The best brushes will have very soft ox hair/white china bristles. Softer brushes have less of a chance of leaving marks or brushstrokes in the poly.

You will probably want to invest in two sizes of brush as well. A larger one that is closer to the width of your floorboards so that you can cover a large area without worrying about it drying before you are finished. A smaller angled brush for cutting in corners without getting the poly on your baseboards and trim.

Step 2: Prepare Your Surface

Using an orbital sander or sanding block, remove any old finishes or paints. This will ensure the best-looking finish on your floors. 

An orbital sander is ideal for covering large areas very fast, but because of the circular shape of the pad, you will need to hand sand along any walls or corners to prevent damaging any baseboards or trim.

Begin with a grittier sanding pad, like 80-120 grit, depending on how deep the blemishes are that you want to remove. Be sure to wipe off excess dust and particulates before your next round of sanding. 

Now sand again with a finer pad, 180-220 grit will raise the grain of your floors and finish buffing out those unwanted scuffs.

Step 3: Clean The Surface

This step is very important if you want to achieve the smoothest and best-looking topcoat with your polyurethane. Leaving excess particles on your flooring will cause a rough and tacky-looking finish that will require starting from step one to fix.

First, vacuum the room thoroughly to remove the majority of sawdust and sand particles from your flooring. Follow up with a damp, not soaking wet, cloth or sponge to pick up what the vacuum missed.

Depending on your choice of water or oil-based polyurethane, you will want to wet this cloth or sponge with water or mineral spirits. Use water or denatured alcohol for water-based poly and mineral spirits for oil-based. This will make applying your first coat much easier by helping to decrease the likelihood of bubbles forming.

Be sure not to use water with an oil-based polyurethane or mineral spirits with a water-based polyurethane because they will not mix.

Step 4: Mix Your Poly And Prep Your Applicator

While your first instinct may be to shake your polyurethane, don’t. This is one of the best ways to cause bubbles to form. Take the extra time to slowly stir the poly, ensuring that it is thoroughly mixed without creating bubbles.

When using oil-based polyurethane, you should mix in 1 part mineral spirits to 3 or 4 parts poly. This thins the urethane and reduces the chance of bubbling.

For water-based polyurethane, mix in 1 part water or denatured alcohol to 9 parts polyurethane to effectively thin the product and help prevent bubbles.

You will also want to soak your brush in mineral spirits or water to further ensure that bubbles don’t form.

Now you are ready to apply your polyurethane!

Step 5: Apply The Polyurethane

Make sure to properly ventilate the room when working indoors. Polyurethane fumes, especially oil-based ones, are especially pungent and shouldn’t be breathed if at all possible. Open windows and turn on a fan if you have one to circulate fresh air into the room. 

If you can’t properly ventilate the room be sure to use a respirator. At the very least, it will save you a headache when you are done.

With a brush, remember to use broad strokes without applying too much pressure.  Always work with the grain of the wood, applying a thin layer to prevent bubbling. It is also helpful to break your project up into smaller sections to prevent the poly from drying before you finish.

With a roller, be sure not to apply too much pressure to ensure an even layer. Don’t roll too fast either, this will increase the likelihood of bubbles or uneven application.

Step 6: Allow Ample Time To Dry

While some brands of polyurethane advertise dry times in as little as 4 to 6 hours, it is always best to give each coat at least 24 hours to dry. This is especially true for less ventilated or exceptionally humid areas. Jumping the gun when applying coats will only result in you having to sand everything down and start over. Take it slow and be patient to avoid any mistakes.

Step 7: Sand Before Applying Your Next Coat

Sanding in between coats can remove any unwanted bubbles or drips that may have formed, so make sure to do this in between each and every coat. Use finer sandpaper (180-220 grit) to avoid over-sanding and making the layer of poly uneven.

Step 8: Apply Enough Coats For The Best Finish

Oil-based products will require fewer coats than water-based ones. Expect to apply 2-3 coats of matte or satin finish and 3-4 if you are using a semi-gloss or gloss. With a water-based poly, expect to apply 4-5 coats since it isn’t as thick or durable as oil.

Final Thoughts

Though time-consuming, applying polyurethane without bubbles can be an easy process. Acquiring the proper equipment, taking the time to properly prep your floors, and resisting the urge to rush the process will make a world of difference in your final product.

Meet your Flooring Expert

Travis McCullough

Travis McCullough

Travis is a lifelong jack-of-all-trades in the construction industry with 20 years of experience in a variety of fields. He’s tackled flooring, carpentry, and everything in between on residential and commercial projects of all shapes and sizes.
Working independently and as part of a crew has equipped him with the know-how to not only complete a project but also teach others the finer points within most building professions. When he isn’t out hanging off of a ladder or crawling around on a roof, Travis spends his time educating people about the construction industry.

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