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Galvanized steel paint shedding? Is there a problem with paint or construction?

The most common surface coating of zinc is the coating of galvanized iron surface, and galvanized iron is widely used in construction and industry. Galvanized iron can protect the steel from corrosion due to its zinc layer as a sacrificial cathode, and has good corrosion resistance under normal circumstances.

In order to prolong the anticorrosive life of the galvanized layer and change the color of the surface, the painting of the galvanized steel structure surface is still the main means for many users who have anti-corrosion and decoration requirements.

However, galvanized surfaces often make people feel disappointed. Why?
We want to make this

The result is this...

Why galvanized steel paint is easy to fall off?
Many users paint on the surface of galvanized iron and steel, often the following problems occur: the large areas of paint fall off. Galvanized steel surface, there is a general problem
First of all, the surface of the galvanized steel is smooth and it is difficult for the paint to adhere to it;
Second, although some oil-based paints or alkyd paints form a film after coating, the oxidation reaction does not stop, and some substances that react with zinc (aldehydes, formic acid, etc.) are produced in the reaction, making the coating and the zinc layer. Some loose material (formic acid zinc) was generated between the two, causing the coating to fall off. The white substance on the back of the peeled coating was zinc formate.
In addition, the surface of galvanized steel generally has a zinc salt. If the surface treatment is not professional, it is difficult to clean.

What kind of paint does galvanized surface choose?
1, epoxy zinc yellow primer
The paint is mainly used for the surface of the alloy, aluminum, zinc surface, etc., high decoration requirements, high weather resistance, corrosion resistance requirements of the field, galvanized steel structure anti-rust primer, the need to support the corresponding topcoat.
General support: epoxy zinc yellow primer + polyurethane topcoat / fluorocarbon topcoat, etc.
However, the zinc chemical composition is zinc chromate and contains hexavalent chromium as a carcinogen. So it's best to avoid.

2, Phosphating primer
The paint can be used as a surface treatment primer for galvanized steel structures. That is, by painting the paint, a surface suitable for coating most paints can be obtained instead of the traditional phosphating process. Some galvanized aluminum surface paints fall into this category. . Alkyd paints, epoxy paints, polyurethane paints, etc. can be painted on the surface of over-phosphorized primer or galvanized aluminum paint.
General professional companies have paints for galvanized surfaces

3, lead-acid calcium primer, zinc powder primer and other non-oxidizing grease coating
Such as: epoxy asphalt coating, chlorosulfonated polyethylene coating

4, this year, some acrylic water-based paint can also have good adhesion.
In short: After starting coating, there must be no substance that can react with zinc and generate adhesion.

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