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The mechanical bonding theory is one of the main mechanisms underlying the adhesion of paint coatings. It involves the liquid paint penetrating into the pores, depressions, or rough surfaces of the substrate; upon curing, this creates an \"anchoring\" effect that results in a strong mechanical bond. At the submicroscopic level, all substrate surfaces exhibit varying degrees of roughness, porosity, or uneven structures. When the coating has good flowability and wettability, it can penetrate fully into these microscopic gaps. Once it has cured, the coating forms a physical interlocking structure similar to \"hooks and anchors,\" \"mortise and tenon joints,\" or \"riveting\" with the surface of the substrate; this effect is known as the anchoring effect. The nature of this mechanical bonding force is friction, which is particularly evident in porous materials such as wood, sandblasted metal, and concrete. Surface roughness directly affects the level of adhesion. Increasing the surface area and texture of the substrate through pre-treatment methods such as sandblasting and grinding can significantly enhance the mechanical bonding strength of the coating. For example, after phosphating treatment, a large number of iron phosphate microfragments are formed on the surface of the cold-rolled steel sheet, and the gaps between them provide numerous physical connection points for the coating. However, if the coating does not penetrate completely, bubbles or voids may remain between the coating and the substrate, leading to the accumulation of moisture and ultimately weakening the adhesion. Furthermore, internal stresses generated during the curing process of the coating due to volume contraction or differences in thermal expansion coefficients can also undermine the stability of the mechanical connection.