Bubbles in a finish are more likely from brushing than from spraying, though it’s possible to get bubbles in a sprayed finish if you have the air pressure turned up real high. Bubbles are caused by the turbulence created by the brush gliding over the surface much more than from shaking or stirring the finish. The problem is worse if your shop is hot or if the finish and wood are at different temperatures.
Because some formulations bubble less than others, especially with varnishes and water-based finishes, you can also switch to another brand, which may reduce the problem. To remove bubbles that have dried in the finish, sand them out and apply another coat.It’s difficult to avoid bubbles if you’re brushing, but you can keep them from drying in the finish. Lightly brush back over the finish to break the bubbles (called “tipping off”), or slow the drying of the finish so the bubbles have more time to pop out on their own before the finish skins over. Do this by adding thinner or a retarder or flow additive. Use mineral spirits (paint thinner) in oil-based varnish and polyurethane. Use a retarder for lacquer, and use a flow additive for water-based finish.