One method that I have used quite succesfully for making armour kits look weathered is too apply pencil lead to the finished product. This gives the model a weather worn look.
Here's the steps I use: (highly scientific)
1. using sand paper grind down a small amount of pencil lead.
2. place lead into a container (plastic works best)
3. (now comes the scientific part) stick your finger in the lead. Not too much lead!!!
4. rub back and forth on the surface of the model until you reach the desired texture.
5. to top off the I use ground pastels chalks to complete the effect I want.
It takes a bit of practice to get it right....I use old kits to practice on
H.M. & G.B.
Shermanfreak
Happy Modelling and God Bless
Robert