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 wood 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, you can sand either type of polyurethane – water- and oil-based.
How to Torch Polyurethane?
If you’re a woodworker, you know that polyurethane bubbles can be extremely tricky to eliminate.
But a small handheld propane torch usually does the trick. 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 it melts.
- 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.
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.
Instead, it’s better to use the sanding method if you need to fix the bubbles in an oil-based polyurethane wood finish.
The only problem with this method is you will need to wait at least three days for the layer of polyurethane to cure. Then, only you can lightly scuff out any bubbles with 120-grit sandpaper.
Once the bubbles are rubbed off, you should apply another layer of oil polyurethane to get a bump-free finish.
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) Stir poly
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.
b) Clean the surface
When you’re ready to apply the polyurethane finish, 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.
c) 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 is recommended to create a more even finish and avoid trapping dirt or bubbles underneath the layer.
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.
Jack Luis is a semi-retired painter who loved painting his clients’ ideas on their walls.
He had worked as a painter for over a decade serving customers in areas such as Charleston, Mount Pleasant, Beaufort, and Georgetown, SC (South Carolina). Today in his free time, he likes to read and write about the newer techniques implemented in his profession. You may read more about him here or get in touch with him here.