Bubbles can be formed by many things, such as improper mixing of the paint, excessive water vapor in the air, small dust particles on the model, or even humidity changes while drying. The only real solution when this happens is to sand and respray where needed.
To help prevent it, don't spray on very humid days, try not to spray outside and immediately bring the model indoors, and wipe down the model pieces with a tack cloth immediately before spraying.
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