
Basically its part of the panel line stage. On most of the panel lines you draw them all in black, then make a copy in white and offset it one pixel vertically and horizontally to give the illusion of a gap in the metal.
Looking at the example above the light is coming from the top right. So for the raised panels I drew them using white lines on the top and right edges and black on the bottom and left. This was done on a seperate layer to the other panel lines so I could easily erase the parts of the panel lines that were below the raised panels.
To aid the illusion I made sure the paint chips did not cross the raised panel edges. The rivets help in this regard too.