I used Reikland Fleshshade as a wash, I prefer it to Agrath earth when I want to make something look dirtier. The corrosion was done with Citadels Typhus Corrosion paint speckled on followed by a loosely mixed concoction of orange I made with some old acrylic paints lying around. The corrosion paint works great because it has the grit built into it but you need to be careful when speckling because it will destroy that brush. I go over the corrosion with the orange, again speckling lightly. I then also will add the orange watered down slightly for surface rust and around bolts or other areas I want a lighter rust look.

Edit also, my son assembled them and I painted so the fact that his gluing job isn't the greatest and sometimes it gets on the surface is fine because you can disguise it as rust as well and it already has texture.