I'd try using photoshop to create a texture and use the airbrush tool to creat patches of grey then blend.soften and smudge them into shape! That's what I have done in the past for creating that worn paint look. Hope this helps!
May the force be with you...always!
A Jedi shall not know anger nor hatred nor Love
The only way to live a good life is to act upon your emotions
A warrior is reborn when he has something to protect