Quick background info: bought the machine new, did two string jobs with ISO Speed 17 poly. The poly string slipped quite a few times when I pulled two strings at the same time to start the main. So I decided to clean the sandpaper-like gripper jaws tonight. Based on what I gathered from the forum, I used 91% isopropyl alcohol and a tooth brush. I took down the bigger part of the gripper jaws. Clearly there were quite some black residues left on jaws. However, I could not clean them away no matter how much alcohol I used and how hard I brushed. Any help?