There are lots of bit maps available on the internet for rust and grunge and scratches. Many are free.
Here ia a link to good quality free textures.
http://www.textures.com/
There are many lessons in the Concept area on digital painting.
https://cgcookie.com/courses/?fwp_division=concept
The Fundumentals of Shading course works on how to use images in blender.
https://cgcookie.com/course/shading-fundamentals-in-cycles/
Here is a good tutorial on Grunge.
http://www.blenderguru.com/tutorials/how-to-grunge/
The farther out one swims the deeper it gets. This is why most projects are a team effort. Hope you found something here of help.
