Optimal Spraying Thickness Standard for Plastic Primer

I. Recommended Core Thickness Range

• Universal safe range: 10 ~ 20 μm

• Minimum thickness: No less than 5~8 μm. A continuous and intact paint film cannot be formed below this standard, resulting in uneven adhesion and poor hiding power.

• Ideal construction range: 12 ~ 18 μm. This range achieves the best balance among adhesion, hiding power, construction cost and topcoat matching performance.

• Maximum limit: Shall not exceed 25 μm in general. Excessively thick primer will induce multiple hidden risks.

II. Defects Caused by Improper Thickness

Insufficient Primer Thickness (below 8 μm)

1. Poor adhesion: Fail to fully wet and anchor the plastic surface, leading to overall coating peeling easily.

2. Weak hiding power: Unable to evenly cover the base material, causing uneven topcoat color and substrate color penetration.

3. Reduced anti-corrosion and solvent resistance: The protective effect on plastic substrates declines, and solvents in topcoat may erode plastic parts.

4. Poor defect covering capacity: Cannot fill tiny surface flaws of plastic workpieces.

Excessive Primer Thickness (above 25 μm)

1. Solvent entrapment: The surface resin cures rapidly while internal solvent fails to volatilize completely, easily causing bubbles, pinholes and wrinkling during curing and baking.

2. High internal stress: Thick film generates large shrinkage stress during drying, which tends to cause coating cracking especially under temperature fluctuation.

3. Poor flexibility: Over-thick paint film becomes brittle and cannot follow thermal expansion, contraction and slight deformation of plastic, resulting in cracking and peeling.

4. Higher production cost and material waste.

5. Altered surface texture and gloss, increasing sagging risk of topcoat.

III. Key Factors Affecting Primer Thickness

3.1 Types of Plastic Substrates

1. Non-polar plastics (PP, PE etc.)

   These materials feature extremely poor inherent adhesion, and special dedicated primer is required. The primer thickness is recommended to approach the upper limit at 15~20 μm to ensure complete coverage and effective adhesion reaction.

2. Polar plastics (ABS, PC, Nylon etc.)

   With good natural surface adhesion, the applicable primer thickness can be thinner, controlled at 8~15 μm.

3.2 Functional Orientation of Primer

1. Adhesion-promoting primer: Focus on improving base material adhesion, suitable for relatively thin construction at 10~15 μm.

2. Conductive / anti-static primer: Needs to form a complete conductive film, subject to the minimum thickness requirement specified by the supplier.

3. Hiding-type primer: For covering dark substrates and surface defects, the thickness can be adjusted to the upper limit of 15~20 μm.

3.3 Final Product Performance Requirements

1. Products requiring high wear and scratch resistance: Proper primer thickness is essential to enhance overall mechanical strength of the coating system.

2. Outdoor application products: Reasonable primer thickness guarantees long-term weather resistance and service life of the whole coating structure.

IV. On-site Construction Operation Guidelines

1. Prioritize supplier technical specifications

   Different brands of primers differ in formula, solid content and construction viscosity. All construction parameters shall be subject to the official Technical Data Sheet (TDS).
2. Follow the principle of thin coating in multiple passes

   Avoid one-time heavy spraying. Adopt the method of two thin coats:

• First coat: Light mist spraying for preliminary surface wetting and priming adhesion.

• Second coat: Spray crosswise to reach required thickness after the first coat is surface-dry.

  This method facilitates solvent volatilization, effectively prevents sagging, and forms uniform paint film with better adhesion.

3. Thickness detection and process monitoring

   Use a dry film thickness gauge to test finished workpieces. Take measurements at multiple positions including flat surfaces, edges and corners to calculate the average value and ensure uniform film thickness.
4. Pre-production process verification

   Complete trial spraying before mass production, and conduct comprehensive tests:

• Cross-cut adhesion test

• Appearance inspection for sagging, orange peel and craters

• Flexibility and impact resistance test

  
  

Summary

1. Take supplier TDS data as the primary standard

2. Adjust thickness according to plastic material and product usage

3. Adopt two-time thin spraying construction technology

4. Monitor film thickness with professional testing tools

5. Verify coating quality through various performance tests