【Haichuan Anti-corrosion Knowledge】Why are intermediate coats mostly epoxy? A bit of curiosity about the industry

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Having sold paint for many years, I have observed a very interesting phenomenon: why, in anti-corrosion systems, is it always the case that the primer contains zinc, while the topcoat is made of epoxy, polyurethane, or fluorocarbon materials? As for the intermediate coat, it is almost always of epoxy type. Epoxy ferrochrome, epoxy thick paste, epoxy intermediate coat… It seems that the industry agreed on this from the very beginning, but strangely enough – almost no one discusses it; few people ask why intermediate coats are almost always made of epoxy Why exactly it? I. Three-layer coating – it’s like a “team”. Let’s not rush to discuss the chemical structure; instead, let’s use a more practical analogy. A typical heavy-duty anti-corrosion system consists of: • Primer: responsible for “holding onto the steel”. • Intermediate coat: responsible for “withstanding the environment”. • Topcoat: responsible for “facing the outside world”. If we compare the anti-corrosion system to a construction team, then: • The primer is what lays the foundation. • The topcoat is what gives the surface its appearance. • The intermediate coat is like the hardworking worker who does all the tough tasks – it has to support both the primer and the topcoat. That’s the problem – the middle layer is the layer that bears the most stress in the entire system. II. What the intermediate coat is really meant to achieve is not corrosion protection. Many people think that an intermediate coat simply means applying another layer of paint to increase its thickness. This is actually only half right. The intermediate coat truly addresses three practical problems: ① Thickening the paint film to extend its corrosion resistance, which is essentially related to one key factor: dry film thickness (DFT). But the primer cannot be infinitely thick: • It is prone to cracking if too thick; • A too-thick zinc-rich primer actually reduces adhesion. The topcoat either cannot be too thick either: • It increases costs; • it tends to sag; • its decorative qualities decline. Thus, the task of determining the thickness falls on the intermediate coat, and there are not many resin systems that allow for a thick application without cracking. And epoxy is precisely the most stable among them. ② Fill in the imperfections of the substrate. Construction sites are never perfect: • Uneven grinding • Rough welds • Micro-pores on the surface • Slight roughness in the primer. The intermediate coat must: ✅ Fill in gaps ✅ Level the surface ✅ Seal it. Epoxy paint has a natural advantage: high solid content + high filler capacity. This is why: epoxy ferrochrome can be applied in thick layers, and epoxy thick paste can achieve 200μm in a single application; with many other resins, it would have collapsed long ago. ③ Being a “relationship coordinator” is something that many people overlook; the primer and the topcoat actually have “completely different personalities”. For example: • Zinc-rich primer → rough, strong inorganic texture • Polyurethane topcoat → smooth, flexible, highly decorative. Direct combination? Problems are likely to arise, and one of the great advantages of epoxies is their excellent compatibility! It can firmly hold the primer in place while also providing a good adhesion base for the topcoat. In simple terms: epoxy acts as the “middleman” in coating systems.