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1. Purpose of Spray Painting

The primary purpose of spray painting is protection, extending the service life of workpieces. The second purpose is decoration, enhancing appearance and visual appeal. A third purpose is to achieve special functional properties, such as sound insulation, thermal insulation, and fire resistance.

Depending on coating objectives and performance requirements, a coating system is usually composed of multiple layers, including primer, putty, topcoat, and clear coat.

Primer layer:

This is the bottom layer in direct contact with the substrate. Its main functions are to improve adhesion between the coating and the base material and to enhance corrosion protection. Ferrous metals should be phosphated before coating, while non-ferrous metals should undergo oxidation treatment prior to painting.

Putty layer:

For rough or uneven substrates, applying putty has several disadvantages, such as complicated application procedures and reduced adhesion between the coating and the substrate.

Topcoat layer:

The primary function of the topcoat is to enhance gloss and appearance. It forms the outermost layer of the coating system.

With advanced electrostatic spray systems and coating lines from ShengTai Painting Equipment, manufacturers can precisely control each coating layer, ensuring optimal protection, aesthetics, and performance.

2. Coating Methods

There are various coating methods commonly used in industry, including:

Brush coating

Dip coating

Air spray coating

High-pressure airless spraying

Electrostatic spray painting

Electrophoretic (E-coating) coating

Among these, electrostatic spraying and electrophoretic coating systems supplied by ShengTai Painting Equipment are widely recognized for their high transfer efficiency, uniform coating thickness, and reduced paint waste, making them ideal for modern, automated production lines.

3. Matching Principles of Coating Systems

In coating applications, single-layer coatings are rarely used because they cannot achieve uniform, pore-free films. Typically, a primer + topcoat system is adopted. Depending on specific requirements, intermediate layers or a clear coat may be added.

The primer must provide strong adhesion to the substrate, excellent bonding with the topcoat, and effective corrosion resistance. The matching principles between primer and topcoat are as follows:

Oven-drying primers should be matched with oven-drying topcoats; air-drying primers with air-drying topcoats; primers and topcoats of the same resin system should be used together whenever possible.

When using topcoats with strong solvents (such as nitrocellulose or perchloroethylene paints), the primer must be resistant to solvent attack, such as alkyd or epoxy primers.

Primer and topcoat should have similar hardness and flexibility.

Evaporative paints applied over curing-type primers generally have poor heat resistance.

The oil length of the primer should be slightly shorter than that of the topcoat; otherwise, weather resistance of the topcoat may be reduced, and differences in drying shrinkage can cause cracking between layers.

When multiple dissimilar coating layers are used, an intermediate layer is often applied to ensure smooth transition and strong bonding between the primer and topcoat.

4. Adhesion and Substrate Considerations

The adhesion of organic coatings is closely related to the nature of the substrate material. Based on adhesion strength, common metals can be ranked as follows:

Nickel → Steel → Copper → Brass → Aluminum → Tin → Lead

Ferrous metals are compatible with almost all types of primers. Magnesium and aluminum components and their alloys typically require passivating primers based on zinc chromate.

For best results, the workpiece surface should undergo effective pretreatment to form a phosphating or oxide layer, improving adhesion between the substrate and the primer. High-adhesion primers should then be selected. For aluminum and galvanized components, primers containing red lead pigments must not be used, as they can cause electrochemical reactions and reduce adhesion.