The coating between the primer and the topcoat is called an intermediate coat. In addition to putty as an intermediate coating, there are also general primers, putty surfacers, and sealing primers.
The function of the intermediate coat is:
(1) Protect the primer layer and putty layer to prevent being bitten by the topcoat;
(2) Increase the bonding force between the primer and the topcoat;
(3) It has a certain filling and leveling ability to eliminate the roughness of the bottom coating (effective for 1o micrometer roughness);
(4) Improve the decorativeness of the coating;
(5) Increase the thickness of the coating to improve the decorative and waterproof properties of the entire coating.
For this reason, the intermediate coating should have good compatibility with the primer and topcoat, and should have good sandability, and a smooth surface should be obtained after wet sanding.
Intermediate coating generally adopts spraying method and automatic electrostatic spraying method. Two coats of surfacer are generally sprayed "wet to wet" continuously, and the thickness of the dry paint film is about 35~4o microns. The intermediate coating is only used for products with high decorative requirements, and the products with low decorative requirements generally do not use intermediate coatings (that is, the topcoat is directly coated on the primer layer).
