Best way for adding scratches and such is by using layers in photoshop.
eg. 1st Layer- Base coat, I tend to use textures from places such as
http://www.mayang.com/textures/ 2nd Layer- The undercoat ("Paint Primer") Just create a plain coloured coat and add some grunginess and small scrated with the sponge or burn tool.
3rd Layer- Paint Layer, this is just the top layer that will be seen the most. Again add some more grunginess and then ur ready to "scratch" the surface.
Now find some good grungy brushes and erase parts of the top layer slowly working down to the next layer. Remember to take your time and get as much detail as you can get into your UV map.
Good Luck,
Blackravent