Preventing Bubbles in Polyurethane Coatings

Excitement comes when you get to the final polyurethane coat as you are almost done with the project and can’t wait to show it off to your family and friends. Or better still, you want to get in touch with the client as soon as possible. But this joy is cut short when you observe some bubbles on the final product. And you can’t help but wonder What is the best way of preventing bubbles in polyurethane?

The common causes for bubbles in polyurethane include improper brush prep and shaking the can. Also, a contaminated surface will deliver unpleasant bubbles. Thus, cleaning the surface, shaking the can, and soaking the brush bristles in recommended solutions will help prevent these issues. For example, use water for water-based formulas and mineral spirits for oil-based polyurethane.

Nobody wants to do all the refinishing work only to end up with bubbles. Thus, please continue reading this piece for more tips on handling bubbles in your finish.

How Do You Apply Polyurethane Without Air Bubbles?

The best strategy that helps you to apply polyurethane without air bubbles involves having multiple thin coats. Next, ensure that you work with a synthetic brush instead of a natural one. Then, dampen it with water and expect a smoother finish.

On top of that, please do not shake the polyurethane can as it creates bubbles. Stir the formula gently and proceed with the application.

Spraying water-based polyurethane is also an excellent way to keep bubbles away. You will deliver a uniform and even finish. Also, you will not have any brush streaks. However, the procedure can be messy. Thus, it would be best to cover the surrounding area to avoid paint spills.

Lastly, you can apply polyurethane without bubbles by adding water to the formula before application. But ensure that you balance the water paint ratio to retain the polyurethane’s durability and quality.

How Do You Fix Bubbles In Polyurethane?

It is not too late if the polyurethane topcoat cures, and you can see some bubbles. The following tricks will help you fix the situations. However, some are more time and energy-consuming than others. Thus, please pick what works for you.

  • Use Steel Wool

Steel wool is an excellent accessory when dealing with bubbles. Begin with a fine OOOO wool as it does not leave scratches on the surface. The wool also makes the finish glossier, and thus, you may have to buff the entire surface to get an even finish. However, you can skip working on the whole region if you intend to add another polyurethane coat.

In addition, consider getting OOO or OO steel wool if OOOO does not give you desirable results.

  • Sanding Smooth

Get high grit sandpaper like 400 grit or more. The accessory polishes off the peace as it sands out any bubbles on the surface. Also, it would help to stick to the recommended sandpaper as coarse one may wreck the finish.

Washable sponges are also an ideal alternative in removing bubbles. But ensure that they are soft and won’t scratch off the topcoat.

  • Restart the Surface

A sandpaper or steel wool may not do the trick if the bubbles are that bad. Here, the best alternative is to use a stripper on the damaged area. Then, stain or paint it before reapplying the polyurethane formula. The procedure may be time-consuming, but sometimes you have no choice.

How Do You Get Bubbles Out of Clear Coat?

Pinhole size bubbles come as a result of the formula trying to escape. Thus, it will be best to allow the paint to escape or breathe before having a clear coat. Also, you can clean the surface thoroughly before applying the paint. The exercise helps to remove any oil and wax deposits that compromise the topcoat’s adherence to the surface.

The nature of the bubbles in a clear coat will determine the remedy. For example, you can fix hardly noticeable sparse bubbling without extensive repairs. Allow the finish to dry properly, and then pierce the bubbles with a fine sewing needle. Next, get a paint scraper and flatten the deflating bubble.

On the other hand, you’ll need to do a complete scrap and start over strategy to fix extensive bubbling. So, examine the surface under the paint and determine the most suitable grit level to use. Then, sand all the paint off the surface.

Besides that, you can choose to reapply the formula in a way that smoothens it. Also, please clean the object with degreasing agents. This exercise helps you to eliminate dirt and debris that compromises the quality of the finish.

How Do You Fix Polyurethane Mistakes?

Polyurethane is an ideal formula for your painting projects. Its advantages set it above alkyd and lacquer vanish as it is easy to apply. The product also forms a hard plastic that offers superior protection to the surface.

However, polyurethane tends to form bubbles that harden on the surface. Also, it does not redissolve after curing. Thus, it would be best to resolve issues on a coat-by-coat basis.

Fortunately, you do not have to give up once you observe bubbles in your work. Check out the following procedure to remedy the situation.

  • Pop bubbles when spraying or painting with the tip of a paintbrush. They appear because of the moisture released by the substrate or the turbulence caused by the brush. In addition, you can avoid the bubbles altogether by brushing slowly and applying the coats at moderate temperatures.
  • Use 220-grit sandpaper to flatten hardened bubbles. Then, take appropriate precautions to prevent them from reappearing and recoat the surface. Also, wait until the temperature drops after flattening the area with sandpaper.
  • Please wait for the drips to cure before scraping them off with a razor blade. You can also sand them with sandpaper. Please note that brushing a tacky finish will leave brush marks. Hence, you’ll end up creating more work for yourself.
  • Scrap the finish while it is still wet with a pull scraper. Then, wash it with a solvent if the finish forms beads. Also, ensure that you use mineral spirits for oil-based formulas and water for water-based ones.
  • Get a sanding accessory, an oscillating tool, or 100-grit sandpaper if the finish turns cloudy after hardening. This defect is not as common with polyurethane as with lacquer. But it can happen if the wood is wet and the conditions are too humid or hot. Also, it is prudent to give the wood time to dry after removing the finish.

Will Polyurethane Bubbles Go Away?

Most of the polyurethane bubbles will go away after five minutes. But this aspect is only true if they appear during application. The bubbles will not go away if they exceed five minutes. Also, they will be visible even after the formula dries.

Thankfully, you can pop the bubbles before the coat dries. This exercise helps you to avoid the need to sand the finish later.

Why Is My Clear Coat Bubbling?

A clear coat finish bubbles when the solvents evaporate at the wrong rate. Also, the wrong temperature reducer, no enough flash time, too much or not enough airflow, and too much material per coat can cause bubbles.

On top of that, you can expect bubbles when the paint coats are too thick, and the top layer cures quicker than the lower one. They accumulate between the two layers because of continued evaporation. Thus, it will help to adopt multiple thin coats.

Luckily, these bubbles can go away. They pop quickly and leave the finish to cure smoothly. The bubbles also go away without leaving craters soon after application. In addition, you can adjust your technique, roller, or paint to minimize bubbling.

How Do You Get White Spots Out of Polyurethane?

Sometimes polyurethane dries on the project and leaves white spots on the finish. These spots mar an otherwise nice-looking paint job. Hence, it would be best to avoid them. Fortunately, you can prevent them from occurring once you identify what causes them.

White spots on polyurethane occur because of two scenarios. One, if you do not stir the formula properly before applying it. And two, if there is moisture build-up. Thus, you can be sure of a perfect finish once you stir the solution well and allow the coats to dry.

In addition, it would be best to prevent the white spots from forming. But if they occur, you can still remedy the situation. Sanding the surface helps to eliminate trapped moisture and thus, delivers a perfect finish. Also, you can touch up the surface with a furniture marker if the spot is small.

Use 100-grit sandpaper for the exercise. It would also be best to use your hand to avoid marring the whole surface. On top of that, you can consider 60-grit sandpaper to knock down stubborn spots. Then, smooth out the marks with 200-grit sandpaper.

Some other tricks to adopt on white spots include:

  • The Mayonnaise Trick. Try applying some mayonnaise on the spot and allow it to sit overnight. The paste draws moisture from below the surface. Thus, the white spot disappears because it is made of water.
  • Touch-up the Small White Spots. The mayonnaise may not draw out all the moisture. Hence, you can consider touching up the white spot with a set of furniture repair markers. In addition, a darker hue will help to cover the white spot. Therefore, no one will notice the blemish.
  • Stir the Polyurethane Before Use. Too much flattening agent, like zinc oxide, causes white spots once it builds up. Thus, it would help to take some time and mix polyurethane properly before application.
  • Wipe Off Excess Stain. You can eliminate the white spots on the surface by wiping off excess stain formula with a clean cloth after curing.
  • Allow Enough Drying Time. It would help to allow the stain or paint to dry enough before further application. Also, some brands may take longer to cure but ensure that the finish cures completely.
  • Use Multiple Thin Coats. It is recommended to use thin coats for your painting projects. They dry faster and facilitate an even finish. In addition, a thick coat may lead to inconsistent drying and thus compromise the final look. The upper layer may cure faster than the lower one and lead to bubbles.
  • Avoid Drips. You can avoid drips by applying polyurethane in thin coats. Thus, you will have to apply multiple coats if you want a thicker finish. Also, brush the area again once you observe any accumulating drips.

How Do You Remove Excess Polyurethane?

You can remove excess polyurethane by using a special stripper. Use a paintbrush and apply it to the wood. Then, wait and do test scrapes within 30 minutes to 24 hours, based on how many coats you want to remove. Finally, use a paint stripper wash once you finish scraping and get rid of any residue.

Can You Use a Torch to Get Bubbles Out of Polyurethane?

There are two efficient strategies for getting bubbles out of polyurethane. They are a torch and sandpaper. The sanding method is suitable for water and oil-based polyurethanes, whereas the torch strategy only works for water-based formulas.

Get a small, handheld propane torch and pass the flame over the polyurethane bubbles until they melt. Also, ensure that you hold the flame for a considerable duration as too much exposure can cause burning. On the other hand, the flame will do nothing to the bubbles if you pass the torch too quickly over the surface.

Besides that, it will help to allow every layer to dry well as recommended by the manufacturer. This exercise will deliver a glassy finish that is bubble-free.

Also, remember to check your timing before using the torch. Please consider having it immediately after applying the top layer and before it cures. It ensures that you do not trap moisture or bubbles when applying the next coat.

How Do You Smooth Out the Final Coat of Polyurethane?

First, get an orbital sander, paint roller, mineral spirits, sandpaper, 6-mil plastic sheeting, wipe-on polyurethane, and synthetic sanding pads. Then, clean the working area. The exercise helps to deliver a dust-free surface and eventually a flawless finish. 

Next, spread the 6-mil poly to keep the floor from spills and drips. Fortunately, you can reuse the sheet several times before tossing it. Also, it makes the cleanup process quick and easier.

Get a lint-free or tack cloth saturated with a solvent and wipe down the surface. This way, you’ll eliminate all dust traces on the surface and enhance the formula’s adherence. In addition, the solvent evaporates quickly, so you’ll not have to wait for long before getting started.

Besides that, it is recommended that you avoid using water when cleaning the surface. It raises the grain, and you will have to sand it again. Also, please consider getting a roller for large and flat surfaces. The accessory helps you apply the polyurethane fast and evenly without leaving puddles, brushstrokes, or thin spots.

The finish will not flatten as soon as you finish applying the formula. However, this occurrence is not a cause for alarm as the finish. The coat will flatten out on its own. You can also roll it out gently and use a raking light to ensure total coverage. 

On top of that, re-dip the roller in mineral spirits. Please put it in a bag, preferably a zippered plastic container, and then allow it to dry after every coat. 

Additionally, finishing both sides at once is prudent, even if the bottom won’t show. Unfortunately, skipping this step can be disastrous as the top can cup, twist, or wrap. Thus, it would be best to coat the bottom and flip it over immediately to finish the top part. Also, do not panic if you see a few fingerprints. Roll the edges and use a dry foam brush to go around them.

The other step is to wipe on the polyurethane for tight or small areas. Also, conducive environmental conditions allow you to apply two to four coats within a short duration. Hence, please consider the humidity and temperature levels before application.

Next, lightly sand the surface with synthetic sanding pads and paper. This way, you can roughen up the surface and rub out hairs, dust motes, and drips. In addition, you may grab 280-grit sandpaper and be more aggressive. The exercise helps eliminate stubborn nibs that are too tough for the pads. Then, gently wipe the surface with mineral spirits and apply another layer.

Finally, allow the final coat to dry in a dust-free area. You can work with a drying booth made from 6-mill poly sheeting. It eliminates dust specks and ensures that the surface is clean as it dries.

What Happens If You Don’t Sand Between Coats of Polyurethane?

Polyurethane’s texture is rougher when you do not sand in between coats. But the flaw is usually not visible to the naked eye. Also, the layers will still adhere to the surface whether you sand or not. Therefore, it is easy to skip the process, thinking that it will not compromise the results.

Even so, it would be best to dedicate some time to sanding the coats. However, although the process may not create a better bond, it helps you deliver a better finish. In addition, it levels dust nibs and other imperfections, and thus you can deliver a smooth finish easily.

How Many Coats of Polyurethane Is Too Many?

You can have as many coats as you want as long as you allow enough drying time between them. However, two to four coats are the most recommended. Two layers are sufficient for areas that are not subject to much human traffic. On the other hand, four polyurethane layers are suitable for floors and surfaces with excess moisture exposure.

Besides that, ensure that you decide the number of polyurethane layers based on the desired finish. For example, four coats may deliver a gloppy and thick finish, which can peel easily. Also, the more layers you apply, the more likely you’ll get a yellow hue.

Lastly, it is best to have polyurethane coats for an oil-based finish. Also, mix the first coat with mineral spirits. They seal the wood and act as a barrier between it and the oil finish.

How Long Do I Have to Wait In Between Coats of Polyurethane?

It is best to allow a polyurethane coat 24 to 48 hours before applying another one. This duration gives the surface enough time to cure and harden. Therefore, you are sure to deliver a better finish,

In addition, the paint job will become a failed product if you recoat the formula too soon. The shrinkage of the bottom coat causes little tipples. Hence, the finish will not be smooth after curing.

Frequently Asked Questions

Readers and woodworkers in general asks a lot of questions regarding the subject. However, here are some of the most prevalent

  • How Do I Know if the Polyurethane Coat is Dry?

There are various methods to use to determine whether the painted surface is dry. For instance, an oil-based finish still requires more drying time if it emits a smell and is tacky.

On the other hand, water based-based coats are not dry if they do not feel cool to the touch. Also, the surface forms powder with light sanding.

  • Is Polyurethane Toxic After it Dries?

Polyurethane is not toxic after curing. It is generally safe, and you can even use it for kitchen applications. However, the curing and drying process releases potentially harmful chemicals. Thus, it would be best to allow the formula to cure in a well-ventilated space. 


A painting project is only complete once the final coat dries and the surface is officially in use. Thus, bubbles in the final coat can cause anxiety as it shows that the work is incomplete. However, it is easy to avoid the occurrence once we understand why it happens. So, let’s engage with the concern…

Why Am I Getting Bubbles In My Polyurethane

The primary reasons for bubbles in the polyurethane finish are painting a contaminated surface, shaking the can, and having air in the brush. However, water-based polyurethane formulas deliver thinner coats. Thus, they produce fewer bubbles than oil-based ones. 

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Tyron Otieno

Tyron is an avid woodworker and writer. He founded this website to help other woodworkers, whether hobbyists or professionals by sharing his knowledge and experiencie after a decade of woodworking.

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