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How to Prepare Galvanized Sheet Before Painting?

Prepainted galvanized sheet is widely applied to various industries, for example, construction, home appliances, agriculture, etc. It uses galvanized steel as substrate, and then be painted with layers of coatings after pre-treatment. This process makes it more corrosion-resistant, durable to use, and good-looking. Wanzhi Steel usually employs coil painting process. While before that, it is also very important to prepare galvanized metal for painting. Below let’s review the pre-treatment process.

PPGI Production Line

PPGI Production Line

Purpose of Pre-treatment

1.To remove the oil and impurities on the steel sheet surface.
2.To make the substrate more adhesive to paint.
3.Further enhance its corrosion resistance and flexibility.

How to Clean Galvanized Steel Before Painting?

The general pretreatment process includes degreasing, washing, surface adjustment (chemical conversion treatment), washing again, passivating and drying. Below are more details.

1. Alkali degreasing. It aims to remove the oil and impurities of the substrate to ensure that the surface is clean. This is the basic step for further painting.
2. Washing (with water). It aims to clean the residual liquid retained on the sheet.
3. Drying the surface. After cleaning the steel, let it rest until all traces of the degreaser have evaporated.
4. Surface adjustment. Its purpose is to improve the coating performance and the adhesion of coating passivating agent and to remove Al, Pb, and other substances on the surface of the galvanized substrate.
5. Washing and drying again. After that, a layer of chemical conversion coating will be formed.

Then the galvanized steel sheet can be painted. Below let’s review the structure of pre-painted steel sheets.

Structure of PPGI Steel Sheet

Structure of PPGI Steel Sheet

Structure of Color Coated Steel Sheet

Simple to say, it contains base metal (cold rolled steel, galvanized steel sheet, or galvalume sheet), chemical conversion film, organic coating. Below is a picture for your reference.

Advantages of Pre-treatment Before Painting
1. Make the Surface Smooth
2. Easy to Paint
3. Enhance Corrosion Resistance

Other Things Need to Know

1.The surface of base metal matters the corrosion resistance of chemical conversion coating. It is proved that the galvanized steel sheet with small spangles and zero spangles is better for making color-coated steel sheets.
2.Washing is the key to forming chemical conversion film. If there are oxides, oil stains, and solid particles on the metal surface, the chemical conversion film will be with breaks. It will influence the corrosion resistance.

White PPGI Coil

White PPGI Coil

Blue PPGI Coil

Blue PPGI Coil

Conclusion

Due to its excellent durability, machinability, and aesthetic appearance, color-coated steel is widely used in many areas. As a top PPGI and PPGL sheet manufacturer, Wanzhi Steel always put product quality in the first place. So we pay attention to every step of the color steel production line, including how to prep galvanized metal before painting. If you are interested in our products, please contact us for a free quote.

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