Alkyd varnish with a polyurethane additive, two-part polyurethane and waterborne polyurethane are all examples of polyurethane finishes that can form bubbles when you brush them. Some bubbles are caused by the movement of the paintbrush, but others may be caused by the release of gas and moisture from the unsealed wood. Many will disappear on their own, but you'll have to sand and recoat to eliminate those that remain after the finish hardens. A few precautions during application can help you avoid this time-consuming extra step.
List of Items Needed
- Mineral spirits
Use a nylon bristle paintbrush for applying waterborne polyurethane and a natural bristle brush for solvent-based products. Soak the brush in water if you're using a water-based product or mineral spirits if you're using a solvent-based one. Shake it off lightly just before you paint.
Brush slowly and evenly, because moving the brush quickly creates turbulence which causes the finish to foam and form bubbles. Apply the material in thin coats.
Go back over an area you've brushed and brush it again lightly with the tip of your brush to remove any bubbles that have formed. Even if you avoided causing turbulence, escaping moisture may form bubbles, and touching them lightly with your brush will pop them.
Add 5 to 10 percent mineral spirits to your solvent-based finish if popping the bubbles with your brush isn't effective. The thinner will slow the drying of the finish and allow the moisture that is causing the bubbles to escape.
Seal bare wood with shellac or a solution of 2 parts varnish to 1 part thinner before brushing the first full coat. This procedure is especially important if the temperature is high or the wood isn't completely dry. Both conditions result in the release of water vapor from the pores of bare wood, which in turn causes bubbles.
Tips and Warnings
- Waterborne polyurethane tends to foam and form bubbles in the container when you agitate it. Avoid shaking the container or transferring the material from one container to another just before painting.
- If a finish hardens with bubbles in it, sandpaper will remove them. Let the finish dry for a full day before you recoat it to avoid having moisture from the previous coat form bubbles in the new one.