What causes fish eyes in a paint finish?

Fish eyes in a paint finish occur primarily due to localized surface contamination in the old finish. This contamination can stem from various sources, such as oil, grease, wax, or silicone residues that were not adequately cleaned prior to application. When the new paint is applied over a contaminated surface, the contaminants repel the paint, leading to craters or "fish eyes." These imperfections disrupt the uniformity of the finish, resulting in an unsightly appearance.

The other factors mentioned, such as the type of paint, humidity levels, and application speed, can influence the overall quality of a paint job, but they do not directly cause the specific issue known as fish eyes. Therefore, ensuring a clean substrate is critical for avoiding this problem in the finishing process.