When selecting a factory, you must prioritize particle size distribution (measured in microns) and material composition. Common sizes range from 50 to 200 microns; smaller flakes provide a subtle shimmer, while larger flakes offer a 'bass boat' sparkle. Ensure the flakes are made from vacuum-metallized polyester or ultra-thin aluminum, as these provide superior brilliance. Additionally, verify solvent resistance, as low-quality flakes may bleed or melt when mixed with high-solvent automotive clear coats.