I have engraved on the top side of a matte black flask with no issue. However, when I try to engrave the back side, it sparks within the center and leaves marks outside of the engraved area (in the center). I tried covering the flask with tape, but that didn’t help neither. The outer edges of the flask engrave perfectly, the center part of the flask leaves a touch of matte in the engraved area. My focus is set to the lowest (center) part of the flask so you would think the center would engrave best and the outer edges not, however, this is not the issue.
How can I stop the sparking towards the center of the flask when engraving? Has anyone had this issue before? As I’ve said ,the front of the flask engraves fine, it’s the back side that I’m having trouble with.