I have had several incidents where there have been paint inclusions on various work that has been painted in our e-coat bath. It looks like small particles that adhere to all horizontal surfaces. P.J.
While running a custom coating operation, I had experienced a similar situation. The amount of "dirt" increased any time there was a line stoppage. We determined that the dirt was in the bath itself and there was also some in the post rinse system. Your challenge will be twofold, one to find the source of the dirt and two to get the dirt out of the system. I will address the second challenge first since it will probably help regardless of the source.
You should review the filtration system(s) with your equipment and paint suppliers. Inadequate filtration due to pumping capacity, filter size (gpm) and filtration quality (micron size) as well as frequency of changing filters can all contribute to dirt accumulation. If you have a holding tank for the bath (for maintenance purposes), is there any filter mechanism available on the transfer piping with appropriate valves for circulation?
To cleanup a dirty tank, I would again suggest getting your equipment and paint suppliers involved. In my experience, our paint supplier loaned us a large multiple-bag portable filter, which we set up with valves and hoses to filter a portion of the bath during production and a higher volume during non-production hours. We started with larger micron bags and then changed to smaller micron bags based on pressure drops. After a couple of weeks we moved the portable filter and set up a diaphragm pump to filter the postrinse tanks.
We eventually added more bag filter capacity and also set up our transfer pump (to the holding tank) with a screen filter. When transferring to the holding tank, a large screen was used to allow for transfer as fast as possible. During circulation and transfer back to the coating tank, the finest pleated screen available was used.
Your second and more difficult challenge is to the source of the "dirt." Your paint or pretreatment suppliers can probably identify the type of "inclusions" by photomicrographs. Be sure to get representative samples from parts or coat panels with a 90-degree bent to represent the horizontal surfaces. If the dirt can be identified by chemical analysis or physical particle shape, it will probably help you identify possible source(s).
In my case, the "dirt" turned out to be paint particles caused by alkalinity being carried into the bath from the pretreatment system. Since a cathodic epoxy e-coat is acidic, if any alkaline solution is carried into the bath a chemical reaction takes place and results in "paint globs" that turn into dirt in the bath. This problem has been addressed in my June 2003 Clinic as causes and solutions to "alkaline drips".