Rather than cleaning the models, I applied a solid coat of "White Knight Squirts" quick dry enamel silver over the top. I think it cost about $9. Preparation is everything, so I masked out the guns and the ammo pipes first with blu-tac. That way I didn't have to repaint them.
Very happy with the results. It was only a few hours work in total so I don't know why I didn't do it as part of the original speed painting Necrons project.
I've also nearly finished painting my Canoptek Moth conversion and was feeling very pleased with myself because I haven't bought any new Necrons since 2013 but, miraculously, had all the models to field a Decurion with 2 Canoptek harvests.
I was so pleased in fact, that I bought a Necron Battleforce to go with it. The spirit is willing, but the flesh is weak. My Night Scythes are a bit shite now and the C'tan Shard is laughable for the points so more core troops might help. I might put a little more care and attention into the new models though. It will have to involve magnets, surely. Maybe some Flayed One kit bashes?