Q. We have to scrap more parts than we want because of craters in paint film. We think this is caused by oil coming from micro pores in the sheet metal. Questions:
A. I have seen film defects similar to those (in emailed photo), and I believe it could be contamination from spray gun air or from conveyor chain lubrication, both of which would be evaporated or burned off by a baking oven. Another common source of surface contamination is spray booth make-up air. Vaporized and particulate contaminants can travel great distances in shop ventilating air and then be drawn into spray booths in make-up air, ending up on unpainted and painted surfaces.
The most important thing for you to do is to identify the contaminant, then eliminate it. The paint process you describe in your attachment seems to be excellent. The substrate problem could be proven by microscopic examination.
Q. We have a dryer and filter at the air compressor, and use a double-cartridge filter prior to the air going to the booth. I have sent samples of the filter to be examined for hydrocarbon contamination, but no contamination was found. The cartridges are also changed monthly.
What sort of takes the coating environment out of the picture is that I have had all of one job show the same defect, while parts from other jobs only 4 ft. away show no defects. All variables (washing process, drying process, coating process, air supply and curing process) are the same except for the metal thickness and the amount of manufacturing being done to the part. We have never witnessed this on material thicker than 11 gauge.
We have executed some experiments on the process in the past, splitting a job into three groups of parts that each went through a different paint process.
Group 1 parts followed the washing and coating process that was described in the attachment sent to you. Group 2 parts were washed through the 11-stage washer, dried for 20 minutes at 270°F and removed from the line following the dry-off oven. Parts then followed the cleaning process described in the attachment. Group 3 parts followed the process described in the attachment but were sent completely around the entire conveyor line twice. The first time through, the Group 3 parts were not painted, however they were subjected to a high, 400°F curing temperature. These parts then were allowed to stay on the moving line for the second pass through the cleaning process.
Test results showed that 13 out of the 15 Group 1 parts had defects and 11 out of 15 Group 2 parts showed defects, while all 15 Group 3 parts were defect-free. We ran this experiment several times and obtained the same results in Group 3 each time. Therefore, I suggested that some parts should be sent around the process twice to remove any contaminate that could be coming out of the steel. This is a costly remedy, however, and is naturally not fondly accepted as a fix to the problem.–S.C.
A. I have never encountered the phenomenon you described when painting sheet steel, although I have seen it many times when painting castings, which are porous. After reading your results, I agree with your conclusions. The samples in test three were defect-free because you forced the offending contaminants out of the micropores during the first pass through the bake oven. My only suggestion is to find another steel supplier.