The cause of rare gray spot contamination defects on car body surface coatingwas investigated in this study. Optical microscopy（OM）and Fourier infrared spectroscopy（FT-IR）were used to characterize these contamination defects from car body surface coatinglayer to pre-coat layer. A series of tests and detections，including man，machine，material， method，and environment aspects，were implemented to check the true cause of the defects. Itwas revealed that the contamination defects were result from pre-coat black slurry penetratingto the base coating layer and formed the grey point defect after long-time stirring. With theimprovement of the pre-coating material formula as well as effective long-term managementmeasures were implemented，the contamination defects were eliminated entirely.