Here is a link to a thread I had a while back and down toward the end there is a description of the method I used. This is after a wash was used to highlight the panel lines as described on Swanny's page and elsewhere in this forum.
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=44330
This is the second plane I have done and the weather is more subtle but done the same way as above except for some white pastel dust rubbed on the wings with finger tips for an uneven faded paint look. It does not show in the photo that well though.
