Used a medium Tamiya file to rough up edges. This is not necessary but I feel it gives depth when the light reflects of the metallic paint.
Used a rotary tool to make bullet holes. Making sure to make the holes bigger at the surface and smaller as they went in. There is no need to go all the way through the plastic (the shadow will look deep enough)
Use a lighter color paint then the color of the plastic to go around most damaged areas.
Used a silver color around the bullet holes.
Used gunmetal inside all of the light green paint and in the inner part of the bullet holes. I used a tooth pick to apply it.
Next I used the brown real touch marker and smeared it with a Q tip. I do this around panel lines and chipped paint to represent rust.
After that I do some really light dry brushing with gunmetal and some dots of real touch brown blended in with the q-tip. If some chipped paint effects don’t look right I clean them up by chipping it away with a tooth pic. Real touch marker can be cleaned up with a white eraser. All of this can be cleaned off with rubbing alcohol.