I have just finished applying panel lines to a model. It is not a true scale airplane and the lines were a bit of an experiment and more for effect than true scale. However they do look good.

I used a Pentel collet pencil. Those pencils will accept different diameter leads. The leads of course come in different hardnesses. I ended up using an "H" lead of 0.50mm dia. For small corner rads I used some old plastic drafting templates to draw around.

Ed S