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The main function of the coating is decoration and protection. If there are flow and leveling defects, it will not only affect the appearance, but also damage the protection function. For example, the formation of shrinkage holes causes insufficient paint film thickness, and the formation of pinholes will lead to discontinuities in the paint film, which will reduce the protective properties of the paint film. In the process of coating construction and film formation, some physical and chemical changes will occur. These changes and the nature of the coating will significantly affect the flow and leveling of the coating.

 After the paint is applied, a new interface will appear, generally the liquid/solid interface between the paint and the substrate and the liquid/air interface between the paint and the air. If the interfacial tension of the liquid/solid interface between the coating and the substrate is higher than the critical surface tension of the substrate, the coating cannot be spread on the substrate, and leveling defects such as fisheyes and shrinkage will naturally occur.　

The volatilization of the solvent during the drying process of the paint film will cause differences in temperature, density and surface tension between the surface and the inside of the paint film. These differences in turn lead to turbulent movement inside the paint film, forming the so-called Benard vortex. Benard vortex can cause orange peel; in a system containing more than one type of pigment, if there is a certain difference in the movement of the pigment particles, Benard vortex is likely to cause floating and blooming, and vertical construction will cause silk lines.　　

During the drying process of the paint film, some insoluble colloidal particles are sometimes produced. The production of insoluble colloidal particles will lead to the formation of a surface tension gradient, which often leads to the generation of shrinkage holes in the paint film. For example, in a cross-linking curing system, the formula contains more than one resin. During the drying process of the paint film, as the solvent volatilizes, the resin with poor solubility may form insoluble colloidal particles. In addition, in formulations containing surfactants, if the surfactant is incompatible with the system, or as the solvent volatilizes during the drying process, its concentration changes, resulting in a change in solubility and the formation of incompatible droplets. There will also be a difference in surface tension. These may cause shrinkage.　　

In the process of coating construction and film formation, if there are external contaminants, it may also cause levelling defects such as shrinkage holes and fish eyes. These pollutants are usually oil, dust, paint mist, water vapor, etc. from the air, construction tools and substrates.　

The properties of the paint itself, such as construction viscosity, drying time, etc., will also have a significant impact on the final leveling of the paint film. Too high application viscosity and too short drying time usually produce a poorly leveled surface.

Therefore, it is necessary to add a leveling agent to help the paint obtain a good leveling by adjusting some changes and the properties of the paint during the construction and film forming process.