What Causes Wood Finish Bubbles? (and How to Fix It)

5 Most Common Causes of Finish Failure

Wood finish (like varnish, shellac, spray finishes, water-based polyurethane and oil-based polyurethane finishes) can completely transform your woodworking pieces, allowing what was once dull wood to come alive with a warm glow.

Beyond adding to the beauty of furniture pieces, wood finish can also help protect them, providing a thin coat that protects them from the elements as well as general wear and tear.

But what happens when your application fails?

You’ll be left with an unsightly piece. Depending on the damage, it might be marred with:¹

• Bubbles: These bubbles are trapped beneath the surface of the finish, causing the wood to be bumpy. This is a common type of damage as it can be caused by a variety of issues.
• Blushing: This is when trapped moisture within the finish causes it to turn white or milky.
• Dust nibs: This happens when dust or debris gets trapped underneath the finish, causing raised bumps on the wood’s surface.
• Brush marks: These are caused by improper application technique.
• Runs: Applying a thick coat of finish, instead of a thin coat, creates drips which dry to form runs.

Besides these problems, wood finish failure will also make your project more vulnerable as these issues breach the barrier of the finish. This can allow further damage to your project, as moisture may seep in and cause warping, cupping, cracking, and so on.

This is why it’s so important we get to the bottom of what causes wood finish failure.

Let’s cover the most common problems woodworkers face, starting with the issue of excess air.

#1 Excess Air

Excess air can lead to uneven drying and bubbles.

If the newly-coated wood is exposed to too much air movement (like a ceiling fan), the finish could dry before it’s had the chance to spread out and level, causing it to dry unevenly.²

Bubbles, on the other hand, typically happen when vapor or air gets trapped beneath the finish’s surface during application.

This can come from the vapor of the finish itself,³ but it can also happen when:

• Air bubbles come up from the pores in the wood when you apply your finish: This tends to happen with open-grained wood like oak or mahogany.⁴
• You shake your can of finish instead of stirring it gently: This causes air bubbles or foam to form.⁵
• You introduce air into your brush: This is caused by overworking the finish, using the wrong type of brush that allows for too many air pockets, or failing to fully soak the brush.⁶

Another common cause of finish failure is when the wood finish is applied with poor technique.

#2 Finish Misapplication

If the finish isn’t applied properly, it could become uneven or streaky.

This happens when:

• You apply overly thick coats: Applying too much finish at once can cause it to dry unevenly or drip to form runs and sags.⁷
• You apply the finish with short, slow strokes: When you apply it in small sections, some sections may have more time to soak it up, causing the finish to look darker or streaky in some areas.⁸
• You overwork the wood finish: You can leave behind visible brushstrokes and streaks when you attempt to brush-over finish that’s already partially dried.⁹ Overworking it can also worsen the appearance of bubbles.¹⁰
• You applied your finish over an incompatible stain or finish: Applying the wrong finish over wood that already has stain or finish on it can lead to problems. For example, just as you wouldn’t want to use paint thinner as the final coat on top of your paint, you wouldn’t want to use a water-based finish over a water-soluble stain—it could smear your previous coat.¹¹

Though these mistakes are common, failure sometimes has nothing to do with the way you apply the finish. Sometimes, it all comes down to the environmental condition of your workshop.

#3 Poor Environmental Conditions

The temperature and humidity of your environment can have a huge impact on how quickly your wood finish dries. This can damage the finish itself—causing it to peel, become blotchy, or experience changes in color.¹²

This happens when:

• High humidity levels or cold temperatures allow moisture to condense and become trapped inside the finish, causing it to turn a lighter, milky color.¹³
• Extreme temperatures and humidity levels cause the finish to go bad.¹⁴
• The finish isn’t given the chance to acclimate to the temperature of its new environment.¹⁵
• You apply the finish to a hot surface, or apply it in direct sunlight: The heat makes it dry out too fast—causing it to be uneven.¹⁶

To make matters worse, the moisture in the air can be absorbed by your wood before you ever apply the finish, causing our next big problem—the issue of excess wood moisture.

#4 Using Wood That Contains Too Much Moisture

Excess moisture isn’t just dangerous to wood finish while it’s in the air—it can also present serious problems when there’s too much moisture in the wood itself.

Firstly, wood finish cannot properly adhere to wet wood.¹⁷ This means, if you don’t bother to check the moisture content of your wood, and you go ahead and apply a finish to it, you might see it start to peel and flake.¹⁸

And if this damage isn’t bad enough, the finish acts as a sealer. Once it’s sealed, the wood will not be able to expel its moisture very well. This will make it incredibly difficult for the wood to acclimate.

With the wood holding onto so much moisture, it may eventually develop water stains, discoloration, and mold.¹⁹ It may also cause warping or cupping to appear in your wood.²⁰

Even if you prevent these problems by carefully monitoring your wood’s moisture levels, you can still run into trouble if you use wood that doesn’t take finish well.

#5 Working with wood species that don’t respond well to finish

Some wood species are easier to apply finish to than others. The most difficult ones are those with irregular or compact wood grain, as this texture makes it difficult to achieve an even top coat.²¹

This is why woodworkers will want to take special care when applying finish to tabletops or chairs made out of birch, maple, poplar, pine, and cherry.²²

But let’s say you fell into one of the five mistakes we mentioned and your wood finish is in rough shape. How are you going to fix it?

Tips for fixing wood finish failure

What if your wood finish is already messed up? Maybe it’s uneven, maybe it’s littered with bubbles, or maybe it looks streaky and discolored. Is there any way to fix it?
Fortunately, yes! There’s almost always a way to fix it (it just might be a little time-consuming).

Here are some general tips for repairing finish failure:

• If your finish is still wet, you can use a dry brush to smooth out uneven coats and bubbles.²³
• If it’s a small problem in one of the finish layers, like bumps from dust, try lightly sanding that layer of finish and applying a new coat.²⁴
• If you’re dealing with moisture issues, the source of the moisture (in the air or wood) must be dealt with before repairs (like sanding and refinishing) can continue.²⁵ To do this, you’ll want to adjust the temperature and humidity of your workshop to match the finish’s manufacturer’s requirements. You can do this by changing the temperature on your thermostat, bringing in dehumidifiers, or increasing ventilation. To fix the moisture of your wood, give it time to acclimate to its new environment.
• If all else fails, try sanding until you’ve reached bare wood. Then, wipe your wood with denatured alcohol and apply a new coat of finish²⁶

How can you keep failure from striking again? Let’s find out!

How to prevent finish failure

Prevention is always easier than fixing or replacing something—the same is true of boards damaged by wood finish failure.

Just think about all the time and effort you’ll waste by sanding off bumps and bubbles, and reapplying finish, waiting for it to dry all over again.

Avoid the headache and get the job done right the first time by following these steps:

1) Investigate your wood: Buy wood that’s compatible with your wood finish. Don’t buy wood that’s been coated in other substances (like wax) that will keep the finish from adhering.²⁷ Avoid buying wood with an open grain (or use lacquer thinner to fill in the wood grain and prevent getting bubbles²⁸).

 

2) Use quality tools (finish and finish brushes): Using quality brushes will help you avoid streaks and bubbles. White china bristle brushes are a great option—they soak up your finish and won’t break down easily (so you won’t have to worry about stray hairs being left behind in your finish).²⁹ Foam brushes are also an option that some woodworkers swear by (just watch out—foam brushes also have a tendency to introduce air bubbles).

 

Whatever paint brush you choose, make sure it’s suitable for the kind of finish you’re using, whether it’s oil-based or water-based.³⁰ (Example: Foam brushes are better suited for water-based finishes while bristle brushes work best with oil-based ones.)³¹ Clean your brush between coats so it doesn’t get tacky—you can use a synthetic brush cleaner or mineral spirits to do this.³²

 

3) Ensure proper environmental conditions: Make sure your workshop’s temperature and humidity match your finish manufacturer’s instructions.

 

4) Check the moisture of your wood: Use a moisture meter to ensure your wood doesn’t contain excess moisture. Visit our online store to browse the quality moisture meters we have to offer.

 

5) Check the temperature of your wood and finish: If your wood or finish is too hot or cold, it could cause the finish to dry too fast or too slow, leading to uneven coats and other issues.

 

6) Soak your brush: Soaking your brush in mineral spirits closes any air pockets in your brush, keeping your finish safe from air bubbles.³³

 

7) Sand your wood: Sanding will keep bumps from interfering with your finish. It will also make it easier for it to adhere to the wood, so make sure to sand before your first coat as well. Many woodworkers recommend starting with a 120 grit sandpaper and working up to a 220 grit sandpaper.³⁴ Remember to sand with the wood grain and clean your wood before applying your next coat for a smooth finish.³⁵

 

8) Stir your finish: If the finish settles, the mixture won’t be fully incorporated and you won’t have an even finish—so be sure to stir after every coat—just don’t shake it or you’ll have air bubbles.³⁶

 

9) Use the correct application technique: Dip your brush halfway into your finish and lightly wipe off the excess on the edge of your container.³⁷ Opt for applying multiple thin coats instead of one or two thick coats.³⁸ Use long strokes with the grain of the wood to avoid patchy coverage.³⁹ If you’re applying the finish with a cloth, thin it by 50% using a solvent (water for water-based finishes, and mineral spirits for oil-based finishes).⁴⁰ Using a cloth is a great option for beginners—it’s easier, but is more susceptible to runs and will require you to apply more coats.⁴¹ Avoid applying a second coat to finish that hasn’t dried yet.⁴² Also, remember to sand between each coat to remove any bumps.⁴³

 

10) Don’t let dust accumulate: Avoid applying finish in a dusty room so dust particles won’t accumulate under your application and make it all bumpy. You’ll also want to wipe your board down with a cloth between sanding sessions to remove sanding dust.

Protect your wood finish from harm

Now you know how to deal with wood finish failure. And better yet, you know how to stop it from failing in the first place.

Now that you know what you need to do, don’t hesitate to obtain the skills and tools you need to get the job done right.

For an accurate moisture meter you can trust to detect excess wood moisture threatening your finish, visit the