Weathered the shoulder guard.
This was done on unpainted plastic.
This was done with a Gundam real touch brown marker, gunmetal paint, and a mix of paints to get a light green.
First I use file were I want some real rough damage (in this case along the outer edge). Then I roughly put down the light green paint with a tooth pick where I want the chipped paint. Then I put gunmetal paint on top of the light green paint trying to stay with in the green paint. If it looks to smooth or you make a mistake you can chip the paint off with a tooth pic when it is dry. I then used the real touch marker on all panel lines that rusty rain water would pour out of. Also used it on the bottom edge of the chipped paint areas. With the real touch I just use a q-tip to smear it downward. Then I did a little dry brushing and a couple of dots from the marker her and there that are then faded in with the q-tip. I will follow up with some step by step pictures.
The effects look a little strong in the pictures but will loose some of the intensity once top coated.