It is a misnomer to say that the line seen on a business strike coin is caused by Die Polishing when they are in fact caused by Die Cleaning. Coin dies are polished to remove scratches in the surface of a die in order to produce a proof-like coin. It is a very skilled and time consuming process. Die Cleaning is an action done to the dies while they are in service to remove debris so the dies can produce a better looking coin. This is a very aggressive action that uses course grit sanding that in turn leaves lines scratched into the dies.