We did comparative testing on a commercial van platform—8 µm vs 15 µm zinc flake, both with the same topcoat. In salt spray and vibration scenarios, there was only about a 10–15% improvement in red rust resistance with the thicker coat. However, most of that came from the consistency of the topcoat. Also, 16 µm caused thread tolerance issues on some of our Class 3 fitments. I’d say focus more on topcoat quality and application control.

If you are curious, we used an inorganic, water-based clear topcoat. Excellent torque control and good resistance to chipping during installation. Honestly, if your topcoat is solid, I wouldn’t bother going past 8 µm—just make sure the application is consistent.