Powder Coating Clinic: Sanding for Repair

Q. We sand aluminum die-cast parts that have minor defects in the powder coating before recoating them. The small areas of dirt, metal blemishes, fibers or similar particles are sanded using 120-grit sandpaper on an orbital sander. After recoating the parts, we often find more little specks and other defects and lines from the sanding disk in the coating. My question is, does this happen because of the sandpaper we are using, or because we sand all the way to the aluminum, exposing the surface and causing it to outgas? 

A. Sanding is necessary for removal of simple particles, scratches and similar defects but care must be taken to avoid a repeat of the defect. You may be getting some outgassing, but it is most likely particles left behind after sanding or cut marks made from the sanding operation. For starters, work on the grit size. The 120-grit paper is fine but it may not be fine enough to avoid lines, especially if you use a motor- driven method. You may need to follow up with a 240-grit to get it smooth. Also, make sure to keep the rotational speed of the device slow enough to control. It is easier to cut deeper at higher RPM. Think of it as finessing, not cutting. Work with your operators to understand the process and the potential pitfalls of too much pressure. Make sure that all sanding dust is removed using clean, dry compressed air or run the parts through a deionized or reverse osmosis water rinse. Better sanding technique, finer grit and more careful removal of dust are the keys.

Originally published in the November 2016 issue.