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
Pro Tip: Since the polyurethane didn’t bubble on the underside of the table, the top surface may be contaminated with a silicone- or wax-based polish. This contamination causes the finish to become pockmarked with little craters called “fish eyes” that are easily mistaken for bubbles. The fix is to strip the finish down to bare wood, wipe it thoroughly with denatured alcohol, restain, and then brush on a couple of coats of dewaxed shellac before applying the polyurethane.
Prep Tip: One common cause of bubbles that many DIYers overlook is improper brush prep. Before dipping a brush into the finish, soak the bristles in mineral spirits for oil-based polys, or in water for water-based polys. Skipping this step can introduce air into the finish and cause bubbling. It’s also worth noting that water-based polys are less likely to bubble than oil-based ones because each coat is much thinner.
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
Pro Tip: Finish restorer Mauro, who has worked on several This Old House projects, notes that improper drying conditions are another common cause of finish defects: “Applying multiple coats and you’re not respecting the drying time in between coats — it has no time to dry, and that creates this texture. And another reason, because we are in New England — cold weather — and the house temperature, if it’s not in the right temperature to apply any finish, also can create this texture.” So beyond brush prep and surface contamination, make sure you allow adequate drying time between coats and work in a temperature-controlled environment.
Technique Detail: Even with the most careful brushing, bubbles will always surface in polyurethane. On a 1998 This Old House Magazine project, finisher Rich Dupee buffed each dried coat smooth with a worn 100-grit screen before applying the next. He also applied polyurethane unthinned, explaining: “If you thin it out, then you’re just putting thinner in the wood. By using it straight, you’re getting more of the linseed oils into the wood.” Positioning yourself across from a window so the light reflects off the wet surface helps reveal any missed spots or emerging bubbles before the finish sets up.
