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This type of coating is most common in the field of painting and repair. For both new and old coating systems, a comprehensive consideration is required. First, comprehensively evaluate the properties of the existing coating and the aging condition of the paint film. Determine the type of film-forming substance (such as resin) in the existing coating system, whether it dries physically or cures chemically, and whether it is thermoplastic or thermosetting. Then, the aging condition of the paint film can be evaluated in accordance with ISO 4628:1-6, such as the degree of bubbling, rusting, cracking, peeling, and powdering, so that appropriate surface treatment measures can be taken. Secondly, based on the properties of the existing coating and the results of the evaluations, a coating with suitable surface tolerance is selected; thereafter, a series of tests are conducted in accordance with ASTM D5064-2016a to ensure good compatibility. The technical principle of this type of coating is relatively simple. It is generally recommended to use coatings with a film-forming mechanism similar to that of the existing coating. Of course, there are also a few cases where physically drying coatings are applied over chemically cured paint films. In short, as long as the new coating is compatible with the existing one, provides good adhesion, and does not cause problems such as delamination, bubbling, or wrinkling, it is acceptable.