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How to Remove Bubbles from Polyurethane

Kevin O’Connor calls on a wood refinishing expert to address unsightly bubbles in polyurethane finishes

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.

“Because you didn’t make either of those mistakes, and because the poly didn’t bubble on the underside of the table, I suspect the top surface is contaminated with a silicone- or wax-based polish. That causes the finish to become pockmarked with little craters called “fish eyes” that are easily mistaken for bubbles.

“The only way to correct this issue is to strip the finish down to bare wood, wipe it thoroughly with denatured alcohol, and restain. Next, brush on a couple of coats of dewaxed shellac, such as Zinsser’s SealCoat. Shellac has the unique ability to block troublesome wood contaminants and still be compatible with any finish applied over it.

“I know that’s a lot of extra work, but in the end you’ll get the results you’re looking for.”

from the Jul/Aug 2017 issue of This Old House