I have never really detailed how I paint my rusty Necrons so I will go over it briefly.
In the top picture you can kinda see the stages in my rust stippling process. On the left side is a brown basecoat (as seen here) with Evil Sun Scarlet stipples over it. On the right side is Wild Rider Red stippled over that. The idea is to leave a little of each color showing through each layer. After a few passes, the brown is completely obscured but it makes for a good basecoat.
At this point, I would apply some Stormvermin Fur in weird and random patches and put some Chainmail over that, but I forgot on this mini. I starting doing this to represent the rusted Necrons repairing themselves but someone at the FLGS said that it could be a robot's idea of what camouflage would be. Either way works for me. I am just happy to paint them in a technique that is different than my other armies.
After applying the repairing metal areas, I would wash the rusted parts with Agrax Earthshade. This kind of turns the metal areas into a muddy mess but they will be touched up later.
In this case, I missed the repairing metal areas and skipped onto the washing. I did this. I just applied the Stormvermin Fur over the washed areas and my Chainmail on top of that. Then I had to apply wash to just those areas and be careful not to leave any wash lines. It turned out ok.
I wanted to paint the Necron technology in a beaten up ancient metal scheme. The concept is that the weapons and parts of the vehicles are built and the rusted portions are actually grown. To achieve the ancient metal effect, I started with a Boltgun Metal basecoat and washed it black to give it a clean but dark color. Then I when right to Mythril Silver as a broken highlight.