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The application of diffusion theory in coatings is primarily focused on interlayer adhesion within the same system and the coating of polymer substrates. Practical applications can be found in the following scenarios: interlayer adhesion between multiple coats of the same system. This is the most typical application of diffusion theory; for example, in home or industrial painting, when water-based wood paints of the same type are applied in multiple layers, after each layer dries, the polymer chains in the new wet coat can dissolve and diffuse into the surface layer of the previous coat. This results in molecular diffusion and interaction between the layers, thereby significantly enhancing interlayer adhesion and preventing separation between the layers. In automobile painting, it is also common to lightly sand the cured primer, so that the resin chains of the subsequent topcoat can spread and embed more easily, thereby improving the interlayer adhesion; this optimization approach is based on diffusion theory. Adhesion of coatings to thermoplastic substrates: When coating thermoplastic substrates such as PP and ABS, coatings containing components compatible with these plastics are used. This allows the polymer chains in the coating to interdiffuse with the molecular chains on the surface of the plastic substrate, thereby enhancing the adhesion of the coating to the substrate. For example, in the plastic electroplating process, the surface of ABS plastic is first roughened; subsequent adhesion of the electroplated layer also relies on the diffusion and bonding between the coating resin and the plastic molecules, further improving the overall strength of the bond. The film-forming process of water-based latex coatings: The final film-forming process of water-based latex coatings involves a diffusion step – as water evaporates and the latex particles move closer to each other, the polymer chains within these particles diffuse and merge with one another, ultimately resulting in a continuous, uniform, and complete paint film. The theoretical basis for this process is diffusion theory, which constitutes a core element of the film-forming mechanism in water-based coatings. In currently available industry literature, diffusion theory is mostly used as part of the adhesion/film-forming mechanism in the optimization of coating formulations and processes; there are few commercial applications in which it serves as the core mechanism on its own, usually working together with mechanisms such as mechanical anchoring and chemical bonding.