How to Fix Bubbles in Polyurethane Using a Torch?

A polyurethane finish may cause bubbles on the wood surface due to many different reasons.

The culprit can be using an incorrect brush, not preparing the brush correctly before starting to apply the polyurethane, or shaking the container of polyurethane instead of stirring it softly.

Taking these things into account and ensuring they are followed correctly will reduce any chances of bubbling polyurethane.

But what if you have already applied the poly, and bubbles have formed on the wooden surface?

Worry not; there are two common ways that you can fix this problem. One way is using a blowtorch, and the other is using sandpaper.

While the torch method is recommended for fixing bubbles on only water-based polyurethane and not the oil-based one, you can use the sanding method on either type of polyurethane to fix the mistake.

How to Torch Polyurethane to Remove Bubbles?

If you’re a woodworker, you know that polyurethane bubbles can be extremely tricky to eliminate. But thankfully, a small handheld propane torch usually does the trick if you know how to use it correctly.

Here’s how you can use a torch method to fix the bubbles in a poly finish.

• Simply pass the flame of a torch over the water-based polyurethane bubbles immediately after applying the topcoat before it dries.
• Keep on using your torch on the bubbles until they melt.
• Be careful not to go too fast or hold the flame in one place for too long, as this could burn the surface and ruin your overall project.

If you plan to put another coat of polyurethane, make sure the previous poly coat is fully dry before applying the next one, following the product’s recommended drying time.

The final layer of polyurethane should be smooth and glossy with no bubbles, using this torch method.

Torch Alternative to Fix Polyurethane Bubbles

If you don’t have a torch or are uncomfortable using one, no problem. You can use a hairdryer instead to fix the bubbles in a water-based polyurethane finish.

Just turn it to the hottest setting and aim the airflow low, but be sure to do this immediately after applying the polyurethane.

Alternatively, you can try using a heat gun. But with this, keep the temperature settings low, not more than 120-140 degrees Fahrenheit.

Will Bubbles in Polyurethane Disappear by Themselves?

Yes… and no. It depends on the size of the bubbles and what caused them in the first place.

Small Bubbles: The Chill Ones

If you’ve got tiny bubbles caused by trapped air during application, you’re in luck. These little guys usually pop and vanish on their own as the polyurethane starts to level out and cure. Within about 10 minutes, they’ll typically disappear without leaving a trace, like they were never even there.

So if you’re just seeing a light fizz right after applying your coat, don’t panic. Just give it a few minutes and resist the urge to poke or brush over them.

Large Bubbles: The Stubborn Ones

Now, if you’re dealing with big, blobby bubbles, that’s a different story. These are usually a mix of trapped air and polyurethane—think of them as mini domes that will dry in place and leave unsightly bumps. These guys usually do not go away by themselves.

To fix them, you will need to let the finish dry completely and then scrape or cut off the raised bubbles carefully. You may also need to sand the area carefully to level the surface before you can get a smooth surface.

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Sanding to Get Bumps Out of the Oil-Based Polyurethane Finish

As I have mentioned, the torch method is not recommended when using oil-based polyurethane because there’s always a risk of the project catching on fire.

To fix the dried bubbles in an oil-based polyurethane finish, it’s better to use the sanding method.

• Decontaminate the surface by wiping it down with a clean, lint-free cloth to eliminate embedded dust nibs that may cause abrasions or premature loading of the abrasive media.
• Abrasively level the finish using 320-grit silicon carbide sandpaper, applying consistent, light pressure to avoid cutting through the polyurethane layer.
• Perform surface particulate extraction using a HEPA-filter vacuum to remove residual sanding debris and fine particulates.
• Apply a controlled recoat of polyurethane via spray application, focusing on maintaining an even film build over the affected areas while avoiding overspray and sagging.
• Finalize the finish by burnishing with boiled linseed oil, which serves to enhance surface sheen, restore depth, and protect the integrity of the cured coating.

The only problem with this method is that you will need to wait at least three days for the layer of polyurethane to dry and cure.

Then, only you can lightly scuff out any bubbles with 120-grit sandpaper to get a bump-free finish with another layer of oil-based polyurethane.

Can You Roll New Polyurethane Finish to Avoid Bubbles?

If you’re tired of your beautiful wood project ending up with a finish that looks like it has chickenpox—bubbles everywhere—then the sprayer is your best friend, not a roller, to fix the issues.

Rolling two-part pour-on polyurethane can trap air and leave you with tiny bubbles that dry into your once-smooth masterpiece. Spraying, on the other hand, applies the finish at high speed and lets it settle onto the surface like a well-behaved cat curling up in a sunbeam—no drama, no bubbles.

For spraying, you’ve got two options:

• Aerosol spray cans are quick, easy, and great for small jobs. Shake, spray, and done.
• Spray gun is perfect for bigger projects, but it does require a bit more prep. Thin your poly, keep the nozzle clean, and spray in smooth, even passes.

Now, before you go full action-hero with your spray gun, it’s important to thin your polyurethane. A good rule of thumb is a 4:1 ratio (4 parts polyurethane, 1 part thinner). This makes the finish glide on like melted butter instead of splatting like pancake batter.

How Can You Avoid Getting Bubbles Trapped in Polyurethane Project?

Preventing bubbles from forming in the first place is always better than fixing them later. So, here are a few tips you can follow…

a) Clean and Prep the Surface

Start by cleaning the surface of your woodwork. The cleaner product you use will depend on the type of poly you will be applying – use denatured alcohol to prep for water-based polyurethane and mineral oils for oil-based polyurethane.

If your surface has holes, cracks, or gaps, those little voids can trap air, and that trapped air will eventually escape as bubbles in your polyurethane finish. To avoid this, use a good-quality wood filler to seal up holes, dents, and cracks before finishing.

b) Thin The Polyurethane Properly

For oil-based polyurethane, mineral spirits should be used as the thinning agent, while water is the appropriate thinner for water-based polyurethane.

When using a sprayer, the recommended thinning ratio is 4:1—four parts polyurethane to one part paint thinner. If you’re applying the finish with a brush or roller, a slightly thicker mix works best, so a 3:1 ratio is more suitable.

• Start by pouring the desired amount into a large plastic container, making sure to measure it accurately.
• Add the appropriate thinner—either mineral spirits or water, depending on the type of polyurethane—and gently mix the solution using a turning stick or paint mixer.
• Stirring should be done carefully and consistently to avoid introducing air, which can cause bubbles.
• After mixing for about 5 to 10 minutes, check the flow and consistency of the polyurethane.
• If it flows smoothly and maintains an even color, it’s ready to use. If it still appears uneven or too thick, stir a little more until the texture is uniform and bubble-free.

If you are using a poly can, gently stir the polyurethane inside the can. Do not shake the can under any circumstances, as this will cause tiny bubbles to form that could ruin your woodwork.

c) Apply Thin Coats with the Right Brush

Oil-based polyurethane should be applied with a horsehair brush, while water-based polyurethane can be used with a nylon brush. I am against using sponge brushes for applying polyurethane, as they often leave behind too many bubbles that are hard to fix.

Before applying the poly, “lay off” the brush to remove any excess polyurethane and avoid dripping.

Dip the brush only halfway into the can to control the amount of product applied to the surface. Also, keep a rag handy to wipe up any drips before they have a chance to dry.

Remember that thicker layers take longer to dry thoroughly between coats. So, applying thin coats of polyurethane with a brush is recommended to create a more even finish and avoid trapping dirt or bubbles underneath the layer.

d)  Let Polyurethane Dry in a Dust-Free Space

To avoid bubbles and imperfections, let polyurethane dry in a dust- and bug-free environment. Products like Varathane Ultimate Oil-Based Polyurethane dry in about 4 hours between coats and fully cure in 72 hours.

You can apply all coats in one day and have a usable finish by the next. Just try to pick a day when you can leave the wooden furniture piece undisturbed, like in a closed garage, for several hours.

e) Apply 2–3 Coats and Lightly Sand in Between

To achieve a smooth, durable, and bubble-free finish, apply at least 2 coats of polyurethane, though 3 coats are often recommended for extra protection and clarity.

Each layer builds up the finish, making it glossier, tougher, and more refined—especially important for high-traffic pieces like tables, chairs, or floors.

Between coats, lightly sand with 320-grit sandpaper to remove any tiny imperfections or bubbles that may have formed. This helps the next coat stick better and creates a silky-smooth final finish. Skipping this step can leave your project feeling rough or looking cloudy—so don’t rush it.

The bottom line

Using the blow torch method between coats is ideal if you want to prevent or fix water-based polyurethane bubbles from getting trapped between coats.

Please do not use the torch on oil-based polyurethane, as it could easily catch fire. The sandpaper method is an excellent alternative in such cases and for those uncomfortable using a blow torch. Be patient, and wait for each coat to dry completely before sanding out any bubbles and applying the next coat.

Overall, follow all the steps and tips correctly, and you’ll be well on your way to a flawless bubble-free polyurethane finish – it’s not as hard to achieve as it may seem.