Who's to Blame?

Question:

We are a metal fabricator caught in the classic finger-pointing situation. We have supplied one of our major customers with parts for many years. Until two years ago, we were also responsible for powder coating with minimal problems using six plus powder coaters over the years. The customer now takes care of the coating. Six months ago, our customer started using a new coater and we have had paint problems since. Currently we have a specific part that has what looks like sand in the paint in selected spots. When we magnified the kernels, the sandy-appearance looks more like craters. Could you recommend a source to send a sample to for analysis and an unbiased opinion, preferably in the Los Angeles area. The problem is most likely one of cleanliness. Is this something the pre-paint cleaning should have removed or remnants of our manufacturing cycle, which we, as the fabricator, should remove? More generically, as a fabricator, what can I expect the painter to clean from my parts (i.e. the residual oils on CRS and P&O from the mill and the dust and grit embedded in same during the manufacturing process, or do we send the parts through a part washer to remove all contaminants)? Any enlightening comments on this subject would be appreciated. W. A.

Answer:

I like long questions since I can give short answers and still fill the page. It would be a waste of time and money to send products to a laboratory for analysis at this time. Craters are most often caused by surface contaminants on the substrate prior to powder coating. Oil in the compressed air used to fluidize and spray the powder will also cause this problem. Lastly, cure oven problems and film thickness control can cause craters, as well. In any case, the problems lie with the powder applicator and not with you.

Any competent powder coating applicator should be able to handle CRS and HRP&O stocks that have gone through normal forming, punching and bending operations. However, some machining fluids can present problems when typical cleaning and pretreatment chemistries cannot handle them. In these cases, the pretreatment chemical supplier is contacted by the applicator to evaluate the type of soil they are dealing with and what needs to be done to clean the part.

Occasionally, we see some fabricators provide some sanding, grinding or blasting of welded areas and laser cut areas to remove smuts, scale and oxides that most powder applicators cannot remove without a pickling operation. Other than that, I would have to say that the problem lies with the powder applicator and not with you.

Pre-washing parts before they are sent to the powder applicator can be more troublesome, since a rust preventative would need to be applied to prevent flash rust on the unprotected part. Some rust preventatives are more difficult to remove than the original soils, so I try to avoid them if possible. Feel free to add my finger to the finger-pointing effort. But be aware that I get to choose which finger to use.