Making a system form a new surface requires giving the system a certain amount of energy. Surface tension refers to the minimum energy required to form or diffuse a surface per unit area, i.e. the free energy per unit area. The forces on molecules in the liquid phase at the surface are asymmetrical with those on molecules in the liquid phase at the interior, and the tendency of this unbalanced force is to pull the surface molecules into the liquid, reducing the surface area of the liquid as much as possible.

   In the actual coating system, due to the existence of solvent volatilization, and in the entire range of the paint film solvent volatilization and the curing speed of the film is different, the surface tension of the entire paint film is not the same. The difference in surface tension between a point and a nearby point is called the surface tension gradient. The liquid will migrate from the area of low surface tension to the area of high surface tension, resulting in uneven film. It can be considered that the surface tension gradient or surface tension difference is the main reason that affects the paint leveling in the actual coating construction process.

   It can be seen that in order to achieve perfect leveling effect, the surface tension difference formed in the film forming process must be eliminated. In other words, the quality of a leveling agent depends on its ability to provide uniform surface tension, and this uniform surface tension will not change with the film (such as solvent volatilization, curing of the film, viscosity increase, etc.).