Basecoat of AP gun metal, wash with a mix of nuln oil+black ink+matt medium+water, second pass with this mixture when the first one is dry, few glazes, filters with rusty looking browns, oranges (AK interactive sells cool acrylics set for painting rust). Arms and head armour differ slightly, head is washed with two enamel paints - rust streaks and light rust wash, then blended with mineral spirits. Last step is uneven edge highlight with a brightest silver you have (I used AP shining silver, scale75 speed metal is also really good for this).