Functions of primer, intermediate coat, and topcoat

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Primer: The “foundation” for bonding to metal. The primer is the first layer in the coating system; it comes into direct contact with the steel substrate and undertakes the most fundamental and crucial role. Core function: Provides excellent adhesion, ensuring that the coating system firmly “grabs” onto the metal surface. Epoxy primers, due to their high content of polar hydroxyl groups and ether bonds, can exert a strong adsorption force on metals, making them the most cost-effective choice. Anti-corrosion barrier: It isolates water and oxygen through a shielding effect. Meanwhile, anti-rust pigments can passivate the metal surface; alternatively, the cathodic protection effect of zinc-rich coatings allows them to be sacrificed in order to protect steel. Middle coat: The “skeleton” that connects the layers. Applied on top of the primer, it is the \"most subtle yet thickest\" layer in the entire coating system. Core function: Increase thickness and enhance barrier properties. The coating is not completely dense; numerous micro-pores can be seen when magnified. The intermediate coat, through a relatively thick layer (usually the combined thickness provided by both the primer and the intermediate coat), significantly lengthens the path for corrosive agents (water vapor, oxygen) to penetrate. Special features: Taking the epoxy limonite intermediate coat as an example, the flaky structure of mica iron oxide allows it to arrange itself in parallel layers like fish scales, creating a labyrinthine effect that further enhances the shielding performance. Even filling: Eliminates minor defects on the primer surface, creating a smooth base for the topcoat and enhancing the final decorative appearance. Topcoat: The \"facing\" exposed to wind and rain. The topcoat is the final layer in the painting system; it is exposed to the external environment and serves both decorative and protective functions. Core functions: protection against ultraviolet rays, weather resistance, and chemical corrosion resistance. Although primers and intermediate coats provide some level of protection, it is very limited – without the protection of a topcoat, epoxy intermediate coats will change color within a week under direct sunlight outdoors, and will start to flake off within a month. Performance metrics: The topcoat must possess excellent resistance to salt spray, artificial aging, and humid heat. The type of resin (such as acrylic, polyurethane, or fluorocarbon) directly affects its weathering resistance. Aesthetic decoration: It provides luster, color, and a pleasant texture, giving buildings or equipment an attractive appearance.