A measure of perceived quality to secure the polishability of new paint systems

The automotive industry continuously strives to reduce their environmental impact. For the paint shop it means to introduce more sustainable paint concepts, while maintaining the production rate and retain the right surface appearance that is crucial for the vehicle’s perceived quality. Today most painted parts are visually inspected and, if needed, manually repaired by abrasive polishing to eliminate spot defects. The repair process consists of one sanding step to remove the defect, and one or two rubbing/polishing steps to restore the surface, but still it tends to be a non-reliable process leaving patterns or clusters of shallow micro scratches seen as three-dimensional shapes moving over the surface when viewed from different angles like holograms. These so called ‘polishing roses’ are hard to detect in artificial light but clearly visible in Sunlight and therefore they constitute a constant quality issue. Accurate polishing procedures in combination with more objective inspection techniques would secure a high surface quality—but what is ‘accurate’? The overall scope of the study was to deepen the knowledge of paint systems to develop test routines for the polishability of coatings already during the development stage, and thereby ease the implementation of new coating systems in production. The study was based on collected process data from professional polishers to define a process window based on key parameters for successful end-of-line repairs of coated surfaces, i.e. strategies minimizing the occurrence of visible polishing traces. A CNC-machine was built up for the purpose to systematically test and evaluate new coating systems and repair procedures. The surface estimation was made by visual inspections as well as by a further developed photometric stereo system providing quantitative images of remaining repair traces.