Hi Matt
There are several ways to paint steel.
My personal favorit for armor is Humbrols polished steel. It works best if applied with airbrush. When dry it looks very flat, but can easily be buffed into an amazing steel look. When polished to your desired sheen, weather it with oil paint and higlight with silver.
Another method I have used is flat black acryllic paint and steel/metal pigments. Paint the part flat black and apply pigment with a finger. the more you rub it, the more it will shine. For a rusty look I first apply dark rust pigments, then metal pigments.
alcladII have a lot of steel finish paints. Haven't tried them my self, but have seen many great results.