For your information: Vallorbe made liner tools are the best around, they cut almost everything, they are made to be use with a hammer. Vallorbe is a Swiss company.
I use a curved liner for backgrounding. They don't slip sideways as a flat graver will.
You must use a bent, and as described above, often an inch has to be cut off to get the curve right for your work. The bend is necessary to avoid marring your work under the back side of the liner. All those lines will wreak havoc on your raised work.