One of the most frustrating flaws in a dried finish is bubbles. The trick is to avoid getting them in the first place. If you can’t do this, you need to deal with them when they do occur.
They are more likely to occur from brushing than from spraying, though they can also occur from spraying if the air pressure is turned up really high. The main cause, whether brushing or spraying, is too much turbulence during application, much more so than from shaking or stirring the finish, which are the causes that are usually cited. I remember being told many years ago that the way to avoid bubbles is to brush very slowly one foot every eight seconds. So I tried it, and it worked! But that’s no way to complete a project. Try it to see what I mean.
Sometimes, you just can’t avoid getting bubbles. Some brands of varnish (including polyurethane varnish) and water-based finish bubble much more easily than others, especially if your shop is hot or if the finish and the wood are at different temperatures. So one solution is to try a different brand. But the better solution is to get the bubbles to pop out before the finish dries. There are several ways to do this.
When you’ve got the time, especially with slower drying varnish, lightly brush back over to break the bubbles. This is called “tipping off.” An even better way is to slow the drying so the bubbles have more time to pop out on their own. Do this by adding thinner or a retarder or a flow additive to the finish. Use mineral spirits (paint thinner) in oil-based varnish and polyurethane. Use a retarder in lacquer. And use a flow additive in water-based finish. It doesn’t take much to have a significant effect.
To remove bubbles that have dried in the finish, sand them out and apply another coat, using one of the tricks above to get better results.
– Bob Flexner
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