How To Get The Bubbles Out Of Polyurethane?

A common issue a lot of people have to face when refurnishing their wood is large bubbles formed after they brush polyurethane layers.

So how to get the bubbles out of polyurethane? 

They can form when you use a roller or even a high-quality paintbrush.

If you do not catch then when they are fresh, they will become a part of the finish.

It is not difficult to prevent making them in the first place, only takes a gentle touch when stirring the polyurethane substance and dipping your brush into it.


What Are The Main Causes Of The Problem?

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There are two main factors that cause a bubbling finish and show brush marks after it has dried:

  • The first is your brushing method, it has to do directly with the way the finish is applied.

Many people expect it would level and remain smooth if it brushes the finish properly.

That is completely false. When applying a finish by brush, the brushing is agitated (means moving the finish around) and produces air bubbles.

Even when you are careful, you can not keep those from forming entirely, but if you brush too hard, you will wind up a whole surface full.

  • Secondly, the more you brush out the finish, the faster the solvent evaporates, and the faster it is mounted.

It will not get enough time to level itself if the finish is set too fast.

There will be little time for bubbles to appear and no brush marks will have time to flow out.

Bubbling issue also depends on three uncontrollable factors:

  • Dust

Before applying coats, dust may be on the surface of your project and settling there as it cures.

Be careful to keep your finishing area as dust-free as possible to avoid extra work.

Hanging plastic sheets around you will help minimize the chance of airborne dust.

  • Moisture

Humidity can also create bubbles, especially in the wood itself.

The moisture can trap the fibers by applying finish on timber not completely dried.

With this moisture, solvents and drying agents used in coats will react to it and evaporate or pool onto the layer underneath your polyurethane layer, forming bubbles.

It is safer to let it dry completely when dealing with wet timber before using any kind of finish.

A moister meter could be required to determine if your board is ready for coating or not.

How To Keep Bubbles Out Of Polyurethane

  • Chemical reactions

When you are not sure what sort of coats were initially used, sanding the project directly into bare wood before the completion process begins is safest.

Test the new finish in an inconspicuous area to ensure that the two do not cause any reaction.

Steps To Apply And Get The Bubbles Out Of Polyurethane:

1. Bubbles in a fresh finish

How To Get Air Bubbles Out Of Polyurethane

Once again, do not shake the polyurethane cane nor wipe your brush on the side of the can.

Both of these actions put bubbles into the mixture and it is likely that you will transfer some of these to the lumber you are finishing.

Whether or not you have bubbles in the can, bubbles in the coat are virtually unavoidable – the friction of the brush against the surface creates them.

Dab lightly the bubbles out of wet polyurethane with the brush tip. Continue to flatten the bubbles gently along the surface. 

2. Bubbles in a hard finish

You may find your finish mocked with small craters or stiff spots.

This happens to board finishers who vigorously pulling the application and then stay off until the finish dries.

The only solution to get bubbles out of dried polyurethane is to sand out the air spots and apply another coat of finish, this time more slowly and carefully.

Check out this tutorial video from paulsDIYsolutions for a bubble-free polyurethane application technique: 

Step 1: Sand the surface

Sand the project progressively with finer grits sandpaper.

Paper with a higher grit number removes the deeper scratches left by paper with lower-numbered grits.

Initial sanding with a medium sandpaper (100 grit) is required in most projects, followed by a fine paper (150 grit) and an extra-fine sanding (220 grit).

Step 2: Remove the dust

Once the board is finished with sanding, you have to remove the dust.

Use a shop vacuum with a soft brush attachment, followed by a wipe down with a clean and lint-free cloth moistened with mineral spirits.

Wipe the surface with a tack cloth as the last dusting step.

Remove The Dust Face

Step 3: Seal the surface

Thin your polyurethane with two parts of it and one part of mineral spirits.

Pour the product into a glass jar and stir the mixture slowly with a flat stick.

Use long even strokes to bend the sealer on with a natural bristle brush (exploded silicone brushes may introduce air bubbles into the finish).

Make sure you catch any runs. Some stains are self-sealing.

Check the information label on your can carefully. You can skip this step if the stain is self-sealing.

Please do not shake a polyurethane can.

You will put air bubbles into the mix that will end up like bumps on the finished layer.

Load your brush by dipping it about 1 into the mixture and apply the board with long, even strokes from end to end.

Catch any drips with your brush and smooth them into the surface.

Keep the wet surface of each pass overlapping until the surface is covered.

Be careful at this step or you have to do everything again to get out of all the bubbles.

How To Keep Air Bubbles Out Of Polyurethane

Step 4: Apply the first polyurethane coat

Brush on the finish right from the can within 24 hours after applying the seal coat.

Do not wipe the brush on the can to get the air bubbles out of your polyurethane. 

Spread the substance with long even strokes over the whole surface.

Do not use too much or you are likely to get runs, just use enough for each layer to get a nice, even finish without dry spots.

Once the surface is coated, brush over it again with the grain, from end to end.

Overlap the strokes so that your coating is uniform. Catch any drips, particularly along the bottom edges.

How Do You Get Bubbles Out Of Polyurethane

Step 5: Shave off the bumps

When the surface becomes totally dry (at least 12 hours), cut any drips away with a razor blade. Be careful to not cut below the surrounding areas.

Step 6: Wet sand the first coat

Get air bubbles out of dried polyurethane by wet sanding, mounted on a sanding block with a 400 grit sandpaper after the first cover is dried for 24 hours.

Dip the sandpaper into the water and remove the blemishes and any dust bumps using a circular stroke.

Use enough water to lubricate the sandpaper so you will not burn through the delicate finish.

Sand deep enough to take away the blemish.

When the board feels smooth, wipe it with a moist cloth and then dry it with a clean, dust-free cloth.

Step 7: Reapply

Apply a second and final coat following the same procedure as the first after 24 to 48 hours.

Once the second coat becomes dry, rub the bumps off (step 5) and sand off the wet board (step 6). Only follow these actions if absolutely necessary.

You need to polish the surface if you have to wet sand the second layer.

Wet Sanding Top Coat

Step 8: Polish the surface if you wet sanded the second coat

Wait at least 48 hours and polish with an automotive rubbing compound.

Dampen the clean, cotton cloth with water and add automotive rubbing compound in a circular motion to the side. Any marks left by the 400 grit sandpaper will be expelled by this compound.

It also reestablishes the gloss within the polyurethane wrapping up.

Let the surface dry after rubbing and apply a smooth, dry cloth to the finishing touch.

If the finish is still a bit cloudy, use the same method for more automotive polishing.

Then, let the surface dry and buff. For even greater luster, a polishing compound shall be used as the final step.

Frequently Asked Questions About How To Get The Bubbles Out Of Polyurethane

How To Get Bubbles Out Of Polyurethane

1. How To Get Bubbles Out Of Water Based Polyurethane?

Water-based poly is less likely to have bubbles than the oil ones because each coat is much thinner.

However, the ways of applying how to get rid of air spots on the board of each polytype are the same.

2. How To Get Bubbles Out Of Polyurethane Floor?

The quality of your floors can be restored by these tips:

  • Hand sanding or scraping:

This works well if only a small portion of the flooring has been affected.

Apply the finish carefully, clean the space, let it dry and add two or three fresh finishing until the color matches the rest of your deck.

  • Scanning:

This method gets rid of the completed layer without removing any extracting any wood underneath. It only works with the polyurethane coat.

This approach allows a new layer to stick to the original one through a buffer to screen the current layer.

  • Sanding machines:

This provides the most effective and permanent hardwood floor restoration technique.

However, it should only be used if entire rooms are plagued with alligatoring.

How To Get Bubbles Out Of Polyurethane Floor

3. Can You Use A Torch To Get Bubbles Out Of Polyurethane?

Yes. Using a torch to pass over quickly to get the bubbles to rise to the side and pop.

This is especially useful when applying for two-part pour-on poly finishes.

4. Any Recommendations On Which Brush Or Kind Of Polyurethane I Should Use?

  • For oil-based poly: Natural bristle brush.
  • For water-based poly: Synthetic brush
  • The roller is not recommended for applying since it has the chance of bringing air spots.

5. How Can I Fix The Old Bubbles On My Veneer Coffee Table?

To repair a veneer blister, slit it open with a razor knife gently.

Then apply the yellow carpenter’s glue under the veneer using a glue syringe or a toothpick.

Press the blister tenderly in order to spread the glue.

Use a wax paper and a flat wood block to shield the repair and then press the blister till flat.


The finishing process can be both the most rewarding and challenging phase to accent and protect your project.

The key to how to get the bubbles out of polyurethane time and patience.

Thin paints, carefully applied and allowed to dry entirely can achieve decent results. Rush to complete a job will inevitably become disastrous so that you have to start over by sanding your work down to bare lumber. Try to get it right the first time so that you will not be sorry later.

Further Reading:

Tags: applying thin ,dry between coats, avoid bubbles, thin coats, denatured alcohol, paint thinner, coats of polyurethane, water for water based

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