Bubbles, spots or a ‘fish-eye’ appearance in the finish is caused by excessive air and foam, which is why we recommend stirring and not shaking colors, and why we prefer cellulose sponges to foam brushes.
This can also be caused by dabbing the color on too thickly and overworking an area with the sponge. Sometimes the bubbles will pop before the finish dries, but if not you have to disperse them more without introducing more air.
If the finish needs to be re-worked, polish and thin the dried bubbles with a damp sponge. Apply another thin layer (or more if necessary) to correct any remaining imperfections, always stirring, not shaking, the finish.