Ive used the excellent Bare Metal Foil Co. panel scribers for years now These have a "hook-shape" tip that they sharpen to a triangular cross section, with a very sharp pointed tip.
For long straight panel lines, believe it or not I use just the very tip of the back end of an Xacto razor saw blade! Why the back end? Because I can "push" it against the work to start the groove, get a guideline groove started, then carefully "pull" it forward toward me, while holding the saw blade at a VERY shallow angle, which lets the groove I've started become a "guide" for getting the long, straight groove that makes most any door line. For working up the "radiused" or curved panel line seen on many car doors, I simply used my Dremel Tool with a grindstone tip, to make a "hooked" or crescent-shaped "cutout" at the very back of the razor saw blade (the edge at the back end of the saw blade that is PERPENDICULAR to the cutting (or saw-toothed) edge of the saw. This allows me to use the very back "teeth" of the saw, which I can carefully scribe in (with repeated passes) as I make the needed curved line.
What if I make a mistake, get a scratch or gouge where it shouldn't be? Simple to fix: Just a little dab of Bondo one-part "Lacquer" spot & glaze putty (let it dry overnight--or use a food dehydrator to force the issue if you have one--I do!), then sand the surface smooth, and rescribe gently.
This does take some practice to master, but I found, years ago, that it works very well--been doing this on model car bodies for about 35-40 years now.