Improper Stirring

• Sometimes the bubbles start right at the beginning. Polyurethane needs to be stirred much like paint must first be mixed before it can be applied to your floor; but it also needs a few minutes to settle, after the stirring, to allow the bubbles time to mix and disappear. If you do not give the polyurethane a couple of minutes to settle -- at least until you no longer see bubbles on the surface -- you'll likely get some bubbles as you paint.
  
  

Improper Painting Technique

• Polyurethane is a fickle finish that needs to be applied just so, or it will react badly and turn on you. The finish should be applied with smooth, slow strokes of the brush or fleece pad. It should not be applied hurriedly nor smeared on like a coat of paint. It needs an even application with measured strokes. Quick movements will cause bubbles to rise to the surface, as will repeatedly brushing the same area.
  
  

Improper Brush

• Some brushes are just not compatible with certain types of polyurethane while other brushes need a good cleaning before they will work. You should always clean your brush by soaking it in mineral spirits and letting it dry before using it to apply polyurethane. You should select a nylon brush for use with water-based polyurethane and a natural-bristle brush for oil-based sealants.
  
  

Improper Cleaning

• Any dirt or grime on the floor, when you apply the polyurethane, will show up as a blemish on the coat and may allow air to be trapped under the polyurethane. This air may bubble up during the setting process. Your floors should be as clean as possible before you begin applying the polyurethane to prevent any blemishes or bubbling.