Quoted from Vermont:So I have a new TMNT Premium and I am guessing I did not clean the balls well enough before firing everything up; I now have gray lines from (I'm assuming) the oil residue from the balls.
that's from all the black metallic dust the cheap flipper coil stops are producing. you can probably reduce this dust and keep your whole machine cleaner by replacing with a better quality stop; i'm sure there's a thread somewhere that goes into deep detail about which are the best to buy.