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.