goes the authentic route as he paints 
Scratch it up!
Finally, here's a little tutorial on making amazingly simple but effective scratches or gouges.
- Start out by marring your lovingly painted mini with a few thin black lines.
- Next, carefully paint a thin white line below the black one.
- Finally, use the base color to clean up any mess and thin the lines a little. Your lines have got to be very thin for this to work and look good!!!
If you are doing scratches on a base color other than grey, use appropriate colors... on green, you might use dark blue and the base green mixed with white, for example.