Shrinkage, usually centered around a droplet or small piece of impurity, forms a circular edge around it. The primer can be seen as the substrate, and the topcoat can be seen as the primer. The edge paint film shrinks towards the center, and there is no paint film on the edge or the paint film is very thin. This phenomenon is mainly related to surface tension. When the surface tension of the coating is high and the surface tension of the substrate is too low, it can cause shrinkage.