If you take a pointy Xacto blade and you grind the edge off until the point turns sideways, you will have a super narrow chisel.
An FPC has a very thin center layer between pours. So you don't want to push the blade into the board, like you might do on a regular PCB. So it's hard to make a clean opening in the coverlay. You might expose more copper than you like when the coverlay tears. And be super careful when you get to the copper. If you scrape all the way thru the copper on accident, you are probably already exposing copper of the other side, because the center layer is so thin. And then you can create shorts.