I have a customer that wants me to engrave some memorials on 105 mm shell casings from a gun ship (super cool!!) but cermark just seems to rub off. Does cermark not work on brass? I thought it did and im running 100 power/1 speed and still nothing. any suggestions? I had also thought about using a ferric chloride etch method on it, but i'm a little rust. Does anyone have some instructions on how to do that?