se a pencil eraser, I am using one I had lying around. The one that worked best is white, branded: Staedtler Mars Plastic and Made in Germany, part number 526 50. It says on its side: “Made for pencil and film” Did that after rubbing the rough goo off with a credit card. Then pencil eraser, and intermittently credit card & rubbing alcohol. Tedious but eraser is crucial. Other erasers worked but not as well as the white Staedtler.