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Bubbles are small, visible to the naked eye or with diameters of several millimeters that appear on the surface of a coating. They represent a heterogeneous system in which insoluble gases enter a liquid under external forces and become separated from each other by the liquid. During the coating production process, bubbles act as interfering factors, causing surface defects in the coated products once they are applied. This not only affects their appearance but also impacts the corrosion resistance and weather resistance of the coating film. Bubbles are usually an indication of insufficient adhesion of the coating; in the case of anti-corrosion coatings, they often represent the first visual sign of inadequate anti-corrosion performance. Bubbles come in different sizes, and they can contain liquid, vapor, other gases, or crystals. The size of the bubbles depends largely on the adhesion of the coating to the substrate surface, the pressure of the liquid or gas inside the bubbles, and the degree to which the coating film is stretched upward in order to balance the adhesion between the coating and the substrate. Based on their shape, bubbles can be divided into bubbles and foam. A bubble refers to a single, spherical, tiny air bubble dispersed in a highly viscous medium. Bubbles produced by solvent-based coatings are mostly of this type. Foam appears when a foamable medium is dispersed; due to the high density of air, a large number of bubbles are formed. As there is a significant difference in density between air and liquid, these bubbles rise rapidly to the surface, forming clusters of bubbles separated by layers of liquid. Most foaming in water-based coatings falls into this category.