My R2-D2 and R5-D4 from the Bandai Model Kits!
The weathering is actually pretty simple, its layering of a different techniques.
First layer is chips using a silver paint with a brush on edges like around R2’s eye. I also do a bit of general chipping with a sponge and silver paint like around R5’s eye. Sponge is amazing for getting a natural look but its not precise.
Next I will go and hit certain areas with a subtle black wash. Most of the metal bits got a black wash but you notice it the most at the plugs on the ends of the tubes and dripping down.
Next is general dirt with a sponge like the more contrasting brown along the bottom of R2’s body and the tops of their legs.
Last is a general wash of whatever colour fits your environment. I wanted these to be Tatooine dusty so I used a light brown wash I made.
Sometimes the order of the wash should change but to me it made sense that dust would be the final layer.
If you have any other questions feel free to ask!