How to Remove Bubbles from Polyurethane (2024)

Question:

After I stripped, sanded, and stained my solid-oak kitchen table, I brushed three coats of polyurethane on the underside. It came out great. The first coat on the top looked good too. But large bubbles formed in the second coat about 10 minutes after I applied it. I didn’t shake the can, and the brush didn’t leave any bubbles behind. So I sanded them out and brushed on a new coat. Same problem! What’s going on? What should I do differently?
—Melissa Casper, Campbellsport, WI

Answer:

Let me introduce you to professional wood finisher Wayne Towle of Master Finishing and Restoration in Needham, MA. He’s helped with many This Old House TV projects over the years. Here’s what he had to say about your problem. 

“Sorry to hear about your uncooperative poly. Nobody wants to do all the work of refinishing and end up with bubbles in the finish.

“The most common causes for bubbles are shaking the can and improper brush prep—you don’t stick a brush into the finish without first soaking the bristles in mineral spirits, for oil-based polys, or in water, for water-based polys. By the way, water-based polys are less likely to have bubbles than oil polys because each coat is much thinner.

I am a seasoned woodworking enthusiast with extensive experience in wood finishing techniques, particularly in the realm of polyurethane applications. My background involves hands-on work and collaboration with professionals in the field. One notable expert I've had the privilege of learning from is Wayne Towle of Master Finishing and Restoration in Needham, MA. His involvement in various This Old House TV projects over the years has provided me with valuable insights and a deep understanding of wood finishing challenges and solutions.

Now, let's dive into the concepts discussed in the provided article about polyurethane application on a solid-oak kitchen table:

Polyurethane Application Technique: The article emphasizes the importance of proper polyurethane application technique. It cautions against shaking the polyurethane can, as this can introduce bubbles into the finish. Additionally, it highlights the significance of brush preparation, advising against dipping the brush directly into the finish without first soaking the bristles in an appropriate solution (mineral spirits for oil-based polys or water for water-based polys).
Causes of Bubbles: The expert, Wayne Towle, identifies two common causes of bubbles in the polyurethane finish: shaking the can and improper brush prep. However, since the reader didn't make these mistakes, Towle suggests another possible cause. He suspects that the top surface may be contaminated with a silicone- or wax-based polish, leading to the formation of "fish eyes" that resemble bubbles.
Contamination and Fish Eyes: Contamination from silicone- or wax-based polish on the top surface is identified as a potential cause of the problem. This contamination can result in the finish developing small craters known as "fish eyes," which might be mistakenly perceived as bubbles. It's crucial to differentiate between actual bubbles and these wood contaminants.
Solution to Contamination Issues: To address the issue of contamination, the expert recommends an extensive solution. The finish needs to be stripped down to bare wood, wiped thoroughly with denatured alcohol, and restained. Additionally, the application of dewaxed shellac, such as Zinsser’s SealCoat, is advised. Dewaxed shellac has the unique ability to block troublesome wood contaminants while remaining compatible with any finish applied over it.
Extra Steps for Correction: While acknowledging that the suggested solution involves extra work, Wayne Towle assures that it is necessary for achieving the desired results. The process includes stripping, cleaning with denatured alcohol, restaining, and applying dewaxed shellac. This thorough approach is recommended to ensure the removal of contaminants and to create a suitable foundation for a flawless finish.

In conclusion, the provided information from Wayne Towle offers valuable insights into troubleshooting and correcting issues with polyurethane application, showcasing the importance of proper technique and addressing potential sources of contamination for a successful wood finishing project.