Q. I have been having major issues with what looks like Swiss cheese in my coating after it has been sprayed. I have heard that a fish-eye-type additive can be added to the coating to help eliminate this issue, but I have also heard horror stories about doing this. Can you give me any insights on this issue?—K.D.
A. I think anyone who has done finishing for any length of time has experienced the issue you are referring to in one form or another. Often, it will take a finisher on a hair-pulling journey trying to isolate the source of the contamination, which typically is the cause of fish eyes. Contaminants can be present in a variety of forms, including oil, grease and hand lotions.
You are right, however, to question whether or not to incorporate additives into your coating materials. Many are silicone-based surfactants, which are designed to release the surface tension of the coating. When the surface tension is relaxed, it helps the coating flow back together when contaminants are present. But you may be introducing another source of contamination.
You did not mention what type of spray equipment you are using, but one area I would look closely at is the air supply. Many types of contaminants can be introduced through your air supply, and it is good practice to make sure the air supply is clean. By this I mean adding, at a minimum, a good coalescing cartridge filter system. Many of these types of filters will trap very high percentages of oil and water before they get to your spray equipment.
Next, look at the type of sandpaper you are using. In my experience, some sandpaper that contains silicone can contribute to this problem as well, especially if the substrate was not blown off or cleaned prior to coating.
If your air supply is clean and sandpaper is not the problem, you should look at external sources. Without over-complicating your spray booth environment, take a close look at any window or door openings, as these could potentially provide a gateway for foreign contaminants from your outside surroundings. As addressed in previous articles, airflow through your spray booth also is important. Maintaining a good positive/negative pressure with an air make-up unit can help provide a cleaner spray operation.
On that same note, I also recommend a good housekeeping regimen within your entire shop, including cleaning overhead lights, pipes and floors with a non-silicone-dust cleaning material. Lastly, if you handle your parts, make sure your hands are clean and lotion-free. You would be surprised how much of an effect this can have on the issue you have described.
If you continue experiencing problems after you troubleshoot your operation, contact your supplier for a recommended remedy. There may be other solutions for correcting surface tension issues without introducing additional foreign contaminants.