Hi, Pong. I haven't much experience with Photoshop, but I use Corel Photopaint 9 for my profiles., which is slightly similar. For weathering, depending on the desired result, I use the 'coloured pencil' and the fine airbrush, using lighter shades of the base colour etc. Fine 'paint chips' etc can be done with the fine HB pencil. Give it a try and see what you think.
Terry.