"Spraying a few coats" is not based on feel, but on Dry Film Thickness (DFT, Dry Film Thickness). Too thin means insufficient rust prevention/coverage, too thick leads to cracking, sagging, and reduced adhesion. Film thickness is the "ruler" of coating quality. This article provides the recommended ranges for each layer and the consequences of losing control.
I. Why control film thickness
Each layer has a functional thickness window: the primer must be thick enough to prevent rust, the basecoat must be thick enough to provide coverage, and the clearcoat must be thick enough to resist weathering. Loss of film thickness control will directly cause paint defects.
II. Recommended Film Thickness for Each Coat (Reference)
| Coating | Recommended DFT (μm) | Too Thin | Too Thick |
|---|---|---|---|
| Electrophoretic coating | 15–30 | Insufficient rust prevention | Embrittlement, waste |
| Primer surfacer | 30–50 | Poor filling | Risk of cracking |
| Basecoat | 15–25 | Poor coverage, color difference | Sagging |
| Clearcoat | 40–60 | Easy gloss loss, not durable | Sagging, orange peel |
Note: The above are common reference ranges, subject to the actual coating TDS, with significant variations among different systems.
III. How to Measure Film Thickness
- Wet film gauge: Measure wet film during spraying, back-calculate dry film, and adjust in real time.
- Magnetic/eddy current thickness gauge: Measure dry film thickness (DFT) after curing; use magnetic type for metal parts and eddy current type for aluminum parts.
- Multi-point averaging: Take multiple points on the same panel to avoid local thinness.
Summary
Film thickness control is a quantitative means of coating quality: each layer has a recommended DFT window. Too thin leads to insufficient rust prevention/coverage and color difference, while Too thick causes sagging, orange peel, and cracking. By real-time verification with a wet film gauge + spot-checking the dry film with a thickness gauge, a closed loop is formed to stably fall within the process window.
Frequently Asked Questions (FAQ)
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