Several ways. One of the easiest is to mix up some "Grimy" black really thin for your airbrush, and use a piece of cardboard held against the panel line you wanna highlight. (Have the plane cleared first for this one in case you goof it up)Hold the air brush WAAAAAAAAY back and mist some paint on lightly. Less is better as it will get darker when you reclear it.

another is to use pastel "Chalk" powder and rub it on with your finger. leave as is and keep your fingers off, you can easily touch it up when needed. Or just clear over the chalk.

Hope this helps, Todd.