I've been exploring a more realistic grimdark style for my more recent projects. To get a nice variation on the large armour panels it's important to have variation in the textures. 

I've used AK Terrains Muddy Ground and AK Corrosion Texture to create what you see in the images.  

If anyone is unfamiliar with the products, Muddy Ground is pretty much synthetic mud and Corrosion Texture has a fine sandy texture to simulate the why metal swells and flakes when it rusts.

I've tried to concentrate the weathering where it would naturally occur.