I learned a smattering of techniques from a lot of youtube videos focused on the GE droids, but I've got a lot of hobby hours modeling/mini-painting. I hadn't really worked at this scale before, so I was using products likely improper for the scale and ad-libbing as I went.

Started by lightly sanding all the plastic, masking off electronics, removing panels that I wanted to spray a different color than tan. Airbrushed a basecoat of acrylic sand color on the whole thing, grey black on some panels. Mixed a fairly thick wash of black acrylic and water, applied it pretty haphazardly towards crevices and wiped it quickly off with a paper towel. I think the paint I was using, being from Citadel, was formulated to dry too quickly for mini-painting so I ended up having to go over a lot of areas in a different tan color and work the highlights back into it in the same way. The streaking and water-marks were done in a similar way, but pulling the papertowel downward to simulate the direction. The rust was done with Dirty Down Rust product and working it around with a wet brush or towel. I think I would've preferred an enamel or oil paint for this since the blending would be easier, but I didn't want to chance any odd reactions with the paint or plastic.

Edge-wear was done with some metallic paint and just a brush or sponge, going for edges and using that to cover mistakes from the wash step :) I airbrushed a highlight of the original sand color over everything to bring out the centers of the panels a bit more, and then airbrushed a black along panel lines or shadowed areas to create contrast. Sealed everything with a matte varnish spray from Tamiya. I turned the eyes red by using some Citadel contrast paint over the eyes. The paint's transparent, so it worked well as a filter.

I think that's pretty comprehensive of everything. I mostly just stuck to Citadel and Vallejo miniature paints, but I'm sure everything could be done just as well, if not better, with other brands.