Snake is right, do not just add black into the panel lines, especially on a lighter colored car, it will be too stark. In my opinion, I have seen some very nice, straight panel lines that had black added to them and it actually detracted from the model. There are several different ways of doing it. I had had good results "pre-shading", (to borrow a phrase from our military modeling brothers). I add the black on top of the primer before shooting color coats. As always, practice on a spare body if you can. Good luck!