Streaks in Coating

Q. We apply a medium-gloss gray powder on our parts at around 3 to 4 mils thickness. Sometimes our operators leave streaks on the parts when they apply the powder, and some areas are darker than others before cure and clearly visible after cure. What is causing the streaks, and how can we fix the problem so we do not get them? J.K.

A. The streaks could be caused by stains associated with your pretreatment process. If this is the case, the streaks will be clearly visible on the part before application and show through the coating after cure. If they are, you need better cleaning, better rinsing and better drainage to avoid stains on the part.

If the part is clean, the streaks are in the film and the most likely cause is application technique. When the gun is moved closer to the part surface it will change the voltage/amperage ratio and more current will flow to the part. The high current in a localized area causes variation in appearance. The operators need to concentrate on maintaining as consistent a gun-to-target distance as possible. If the gun has to be moved in close to the part to get coverage, they need to take care to avoid excess film build and texture. Keep the flow rate down and the amperage down.