Automotive paint homogeneity revisited: A shift from intra-vehicle to intra-part variability.

It is well established that automotive paintwork is not homogeneous across an entire vehicle. This heterogeneity can be problematic during forensic comparisons. However, the homogeneity of the paint system within a single automotive part has not yet been extensively studied. The homogeneity and intra-variability of the paintwork were assessed for 28 automotive parts. For each part, five to seven paint fragments were examined with microscopy using brightfield, darkfield, crossed polars, and fluorescence. The layer sequence, number of layers, individual layer thickness, and total paint thickness were assessed. In all cases, variations in thickness and/or in the sequence and number of layers were observed within the same automotive part. This was also the case for Original Equipment Manufacturer (OEM) systems, where variations in layer thickness were observed. For plastic parts, the clearcoat was more likely to exhibit the greatest variability. For metal parts, thickness variability was generally more pronounced for the primer surfacer and clearcoat, while the primer (e-coat) remained the most uniform. Automotive parts with repaints/repairs showed a higher variation in total thickness. These results emphasize the importance of collecting multiple reference samples as close as possible to the damaged area.