This week, PTING had the opportunity to participate in the sampling of a C5 product and, through comparative analysis, demonstrated the importance of mastering pre-treatment methods in coating anti-corrosion. (Note: C5 is a high-standard anti-corrosion coating grade under the ISO 12944 standard, applied in high-humidity, high-salinity environments such as offshore, coastal, and underground areas. The coating system consists of a zinc layer, intermediate paint, and topcoat.)

Here are three samples produced by PTING. The client highly praised the appearance of our samples, though from our perspective, appearance is secondary.

This is a section of a sample from another coating anti-corrosion supplier. As shown in the red circle, there is already peeling, indicating inadequate adhesion.

A close-up comparison reveals that the competitor’s sample (upper section) has insufficient coating coverage at the chamfered edges. In contrast, PTING’s sample (lower section) has a smooth coating surface, while the competitor’s sample feels rough.

The client, lacking in-depth knowledge of coating technology, initially judged the coating quality based solely on surface appearance. We strongly disagree with this approach. PTING consistently emphasizes that what matters is not the appearance but the internal quality. At our request, the client conducted destructive tests on the coatings.

First, a blade was used to scratch the competitor’s sample. As shown in the image above, the coating peeled off in large sections. The same test was then performed on PTING’s sample, with results shown below.

The comparison clearly shows that the competitor’s zinc layer has largely peeled off, while PTING’s sample only suffered damage to the topcoat, with the intermediate paint remaining intact and the zinc layer fully protected.

From these destructive tests, it is evident that the materials used by the competitor were identical to ours and of acceptable quality. Why, then, was there such a significant difference in adhesion? The answer lies in the details of surface preparation. From the areas where the zinc layer peeled off, it is clear that the competitor used shot blasting for substrate treatment. While shot blasting is more efficient and cost-effective than sandblasting, it is unsuitable for thermal spray zinc applications. Under high magnification, sandblasted surfaces exhibit sharp peaks and valleys, whereas shot-blasted surfaces are smoother. How can a smooth surface provide a strong grip for metal coatings? Moreover, thermal spray zinc requires a higher surface roughness (RZ), which shot blasting cannot achieve. Thus, the choice of pre-treatment method fundamentally determines coating quality.

Additionally, why was the competitor’s paint coating rougher than PTING’s? The reason is that the zinc layer itself is rough, with numerous tiny pinholes. These pinholes contain air, which can cause bubbling if paint is applied directly. Therefore, a very thin layer of paint must be pre-applied to eliminate air from the pinholes before subsequent paint layers are sprayed.

Thermal Spray Zinc Coating Surface

Beyond pre-treatment methods and pre-coating for zinc layers, other critical details must be meticulously managed, including sandblasting angle, surface roughness, zinc coating thickness, paint thickness, and air temperature/humidity. In coating anti-corrosion, details determine the success or failure of the coating!