Mechanisms of Compounding and Cryogenic Finishing in the Repair of Automotive Paint Finish Systems

Spot defect repair systems for automotive paint finishes have enjoyed a great deal of success in domestic and foreign automotive markets over the past ten years, saving both time and money by eliminating the need for scrapping or refinishing defective parts. These systems typically involve sanding with a fine grade coated abrasive to remove the defect followed by compounding to remove sanding scratches. While compounding has been shown to work extremely well on rigid enamel base coat/clear coat finishes, it becomes less effective on newer, flexible two-component polyurethane (2K PUR) coatings. These low glass transition temperature materials exhibit elastic behavior under ambient conditions and are resistant to compounding using conventional methods. However, it has been found that compounding is effective on these materials if they are cooled below their glass transition temperatures. This process is known as cryogenic finishing.

This report describes spot defect repair systems and their evolution over the past few years. A discussion of surface microscopy techniques used in this study is given as well. The authors will use experimental data and microscopic analysis to describe the mechanisms involved in compounding and explain why cryogenic finishing is so effective. Finally, the authors will suggest an additional step toward optimal surface finish refinement which may augment cryogenic finishing systems currently in use.