Extinction principle

Roughness of the coating surface
The gloss of an object's surface is closely related to the roughness of the object's surface. When light hits the surface of an object, some of it is absorbed by the object, some is reflected and scattered, and some is refracted. The less rough the surface of an object, the more light is reflected and the higher the gloss. Conversely, if the surface of the object is uneven, more light is scattered, resulting in a decrease in gloss.
The roughness (h) of the surface of a bright object that can be perceived by human vision can be calculated according to the formula: h = λ/cosα using the knowledge of micro-surface theory, where input is the wavelength of human light; α is the incident angle. For example, when the incident angle is 60°, h=1.1μm can be calculated. When the surface roughness h of the object is greater than 1.1μm, it will show unevenness and reduce the gloss.
film forming process
After the paint is painted on the surface of the object, it is cured to form a film by the evaporation of the solvent (for solvent-based paint). The formation process of the coating film is very important to the surface roughness and gloss of the coating film. In the wet film stage, the volatilization rate of the solvent is controlled by the diffusion of the solvent on the surface of the coating film. When the volatilization rates of the various components of the solvent are not very different, it is possible to obtain a high-gloss surface; When the volatilization rate of the film stage is not the same, it will make the polymer molecules tend to form curls, or even precipitation, becoming particles or agglomerates of different sizes, and the surface of the coating film will appear uneven. In the dry film stage, the volatilization rate of the solvent is mainly controlled by the diffusion of the solvent in the entire coating, and also affects the roughness of the coating surface. In addition, during the formation of the coating film, with the volatilization of the solvent, the coating film will become thinner and shrink, and some suspended heavy particles in the coating will be rearranged on the surface of the coating film, causing the surface of the coating film to be uneven.