Occasionally I will get a black film on the inside of the swirl ring. This usually occurs at the end of our season (we are only open for the spring and summer). I suspect its because the filters in my air system are ready to be changed and also because the consumables need to be replaced again. So I just clean the swirl ring with a solvent and it seems to help.
Also, I noticed that we get more stuckies when we are running the 65A nozzles instead of the finecut. Don't know why this is but it might have something to do with air pressure. Our low pressure switch is set at 90 PSI and it's probably less at the Pmax when the compressors cycle on. I am going to raise it next season to 100 PSI and see if this helps.
Willy