Why is an intermediate coat of paint used on the pipes of some equipment, while others only have a primer and a topcoat?

It’s probably because the demands are different, so their prices vary as well

First, we need to understand that the principles for corrosion protection on metal surfaces involve \"primer, intermediate coat, and topcoat\". Its function is as follows: Primer: Its main role is to provide rust protection; generally, primers contain a high amount of rust-inhibiting pigments, which gives them good rust-resistant properties. Uniformly cover the original color of the base. Additionally, the adhesion of the primer is generally good. Topcoat: It mainly provides various chemical resistances, with good properties such as resistance to acids, alkalis, and salt spray. Intermediate coat: It is usually a scale or limonite intermediate coat; due to the flaky fillers, it has good impermeability. Under specific conditions, it can extend the service life, reduce costs, improve the adhesion between the primer and topcoat, and enhance the decorative qualities of the topcoat. When preventing corrosion in equipment pipelines and similar structures, we distinguish between internal and external corrosion protection. For external corrosion protection, we* generally design it for the outer wall to resist corrosion under conditions such as exposure to atmospheric gases. Under such conditions, the advantages of the intermediate coat can generally be utilized to achieve the best economic benefits for the anti-corrosion coating. For example: the outer wall of oil storage tanks, the use of steel structures in the atmosphere, and so on. The inner wall is corrosion-resistant; metals are often in contact with liquids, pressurized gases, corrosive substances, as well as various temperatures and pressures. If an intermediate coat is used under such conditions, the results will not be good. Mainly, in such a corrosive environment, the corrosion resistance of the intermediate coat is relatively inferior to that of the topcoat. Since the coating serves as an anti-corrosion layer, it is not completely sealed and thus contains pores; the intermediate coat has a higher porosity than the topcoat. Therefore, there is a risk of premature failure of the coating. So, generally, when designing, only the primer and topcoat are considered. For example: the inner walls of oil storage tanks, heat exchanger tubes, oil well pipes, etc. Therefore, in general, for equipment in the atmosphere that requires corrosion protection, design requirements can be established for the bottom, middle, and top layers. As for the metal surfaces of equipment in contact with liquids, etc., they are designed with primer and topcoat.