Identifying and repairing defects in rough grinding and metal finishing leads to consistent, attractive Class A surfaces.
In automotive assembly plants, there is sometimes a misconception that the paint operation is the most critical since it is the final process everyone sees. This is only partially accurate. Body-in-white is the first step, and it sets the foundation for e-coat, primer, basecoat, and clearcoat. If manufacturers don’t identify and remove defects in body-in-white, the paint processes can accentuate imperfections or create paint defects. Repair costs are compounded when defects are not identified and removed only in the painting processes.
Body-in-white consists of two critical machining processes: rough grind and metal finishing. Rough grinding can include blending mismatch and removing or leveling metal flash, tabs, and excessive tack weld. Metal finishing produces the final finish for the rough grind areas and removes and finishes minor defects such as in-dings (above or below the surface) due to stamping or handling. In-ding areas are typically found on hoods, roofs, doors, fenders, quarter panels, and trunk lids – critical aesthetic components that are in the driver’s line of sight.
Under and over-processing
Achieving Class A surfaces in automotive applications is a typical Goldilocks situation – too much or too little repair leads to problems, so companies have to get the results just right.
If the defect is under-processed, file marks or circular scratches not completely removed will telegraph through the paint layers. This is a common defect identified in the e-coat process, and they can be removed at a relatively low cost.
Over-processing defects can lead to a mar – a smooth and shiny area on the surface. The mar must be manually scuffed with a fine grit abrasive in an e-coat or primer application. Otherwise, the basecoat (paint color) will not cover the sanded area identically to the non-sanded area, resulting in off-color flaws. Since basecoat and clearcoat are the final two paint processes, off-color flaws can become major defects that don’t get identified until final inspection. This requires the vehicles to be taken off-line to be repaired – a costly process due to the labor, materials, and time.
The start to most body-in-white repairs, rough grinding is typically completed with a right-angle sander at 6,000rpm using a resin fiber disc (F980) or a D-weight paper disc (A995), depending on the severity of roughness. Next it is blended with a random orbital sander, also known as a DA Sander, at 12,000rpm with a 3/16" orbit using a B-weight paper disc (A275/A975).
When the rough defect process is completed, the area is finished with a random orbital sander (DA) with a 3/16" orbit to remove file marks or circular scratches. The random orbital sander should be started on the vehicle to avoid gouging. It should be used with an uninterrupted motion, applying medium-to-intermediate pressure to remove file marks or circular scratches, and feather outward. Defects should never be removed at an angle to the surface, because it will cause severe gouging or deep, wild scratches.
As with any grinding, blending, or finishing operation, all aspects of the process are imperative to the right finish. The right-angle sander, type of file, random orbital sander, air supply, air hose, back-up pad, and the abrasive itself all play an integrated role. Any one of these on its own and improperly selected can cause a defect to the workpiece. Right-angle sanding or random orbital finishing processes are critical, manual operations where operators must rely on their ability to remove surface defects and produce consistent finishes at high production line rates.
The foundation of the paint process in a prime automotive plant commences in the body-in-white shop. If the defect removal process is not properly executed – to account for under- or over-processing – the end result will be a paint defect.