It's never a good idea to do those harsh, solid black panel lines, for a couple of reasons. First, panel lines in real life are never 100% black. They appear as a darker shade of the body color. Second, by making the lines black, they are way to obvious, and out of scale.
On a real car the panel lines are much more subtle, and in the case of where the front fender caps attach to the fenders, that seam is almost invisible on the real car...
A better way to do panel lines: If you use an airbrush, scribe them deeper on the bare body, then paint the body as usual. Then add the panel lines in black... and then shoot another coat or two of color that's been thinned down. You want those final color coats to be translucent. That way the stark black of the lines is toned down so that they look much more in scale and realistic. If you don't use an airbrush, just scribing the lines deeper before painting will probably work to give you more realistic panel lines. But those stark black lines aren't the best way to handle things.