The best way to gut a thin line in metal is to use a rotary tool with a thin cutting disc. First, secure the metal in a vise or clamp. Then, use the rotary tool to cut along the line, being careful not to cut too deeply. Once the line is cut, use a pair of pliers to remove the excess metal.
This technique is especially useful for cutting thin lines in sheet metal, as it is precise and does not require a lot of force. Additionally, it can be used to cut a variety of metals, including steel, aluminum, and brass.