Bubbles in polyurethane

Likely causes

The causes the most sources give come first.

1. Shaking the can mixes air into the finish.

   Two sources: Minwax (Sherwin-Williams) and Rust-Oleum (RPM) , sources [1, 2]

2. Brushing it out too much (overbrushing), or putting it on with a roller.

   Two sources: Minwax (Sherwin-Williams) and Rust-Oleum (RPM) , sources [1, 2]

How to tell which

None of the sources we read says how to tell these causes apart. Check each one against how you finished the piece.

The fix

1. Once the coat is dry, sand the flaw level with fine paper, wipe off the dust and put on another coat. The makers name 220 grit (Minwax), 220–320 (General Finishes) and 400 for sprayed lacquer (Target).

   3 sources agree , sources [3, 4, 5]

Next time

1. Stir the finish; don’t shake it. Minwax says a can that has been shaken should stand until the bubbles break.

   Two sources: Minwax (Sherwin-Williams) and Rust-Oleum (RPM) , sources [1, 6, 7]

2. Lay the finish on and leave it: it levels by itself, and brushing it back and forth works air in.

   3 sources agree , sources [1, 8, 9]

3. Use a synthetic-bristle or foam brush for water-based polyurethane.

   3 sources agree , sources [1, 4, 6]

Often comes with

• Brush marks
• Dust nibs

The finishes it applies to: oil-based polyurethane, water-based polyurethane, wipe-on polyurethane, spar urethane. Every other problem: the finish troubleshooter.