Tank anti-corrosion coating

That’s probably not rust prevention; the primer wasn’t applied properly.

The base coat has been applied everywhere, so there’s no way rust will occur. Those are the pitting left over from rust removal.

As the moderator said, ST3 is not a sandblasting and rust removal standard. You’re right, why are the requirements for the paint such high? Sandblasting is mainly used for rust removal and increasing surface roughness, but there are standards for this. It’s likely that the hardness of the sand particles you chose is too high, and the particles do not meet the standards

Thank you very much for pointing out the error; it’s not St3, it’s Sa3. I used to be in charge of pipeline anti-corrosion work, involving manual rust removal, with a requirement of St3. By the way, if yellow rust appears on a painted surface after it rains, is it possible to simply cover it with more paint?

If rust appears after being exposed to rain shortly after applying one coat of primer, it indicates that the primer contains many pores, which allow rainwater to penetrate through those pores and cause corrosion of the substrate, resulting in yellow rust. The presence of yellow rust represents a potential problem in the future; under normal circumstances, the areas affected by rust should be polished to St 3 level using a sanding disc, and then primed again.

Reaching SA3 grade is very difficult. The sandpaper-like texture mentioned by the original poster is normal; it results from the roughness created by sandblasting. If rust removal is carried out using power tools or wire brushes, as you mentioned, such a texture will not occur – power tools actually make the surface smoother as they polish it. In fact, this is not good for corrosion protection, as the smoother the substrate, the weaker the adhesion between the anti-rust primer and the substrate. Furthermore, the appearance of yellow rust is certainly due to the excessive roughness caused by sandblasting, which results in the primer not covering those areas properly. In the areas where yellow rust appears, the thickness of the primer is definitely not sufficient as per the design requirements. The yellow rust must be removed thoroughly using appropriate tools before any further painting can take place; otherwise, the rust in those areas will continue to develop, and problems will surely arise in the end!

What was said upstairs is correct – the SA3 standard is a very high one; ordinary grinding cannot meet it. High-pressure sandblasting is required, and even the type of sand used has to meet certain specifications. For rust removal at this level, the paint used must be expensive, and its corrosion resistance should also be high. After applying the primer, it may be possible to see the underlying layer (which is transparent); if it turns yellow, it indicates that rust has formed, and the surface needs to be treated again.

Sa3 is specified in GB8923 and ISO8501-1; it represents the highest level of rust removal standard among these. It involves rust removal using methods such as sandblasting or shot blasting, and is also known as \"white-metal visible\" rust removal. St3 refers to rust removal done manually or with power tools – these are two different methods. Generally, sandblasting for rust removal results in a relatively high roughness level of around 40–75 microns. When a primer is applied, gaps can still exist, and rust will reappear after it rains – this is almost unavoidable. Therefore, it is recommended to apply the primer in two stages, as this will **reduce the number of gaps. For areas where rust has already appeared, they should be treated to reach St3 standard before applying the primer again; there is nothing else that can be done in such cases

As a rust removal standard for Sa3 grade, it is normal for the surface to have pitted areas, as the intensity of sandblasting is high, which inevitably results in uneven pits on the surface