-boltgun metal base colour ontop of black primer
-slop on some badab black wash
-highlight with a mix of badab black and boltgun metal
-increase highlights with boltgun metal
-wash lightly with hawk turquoise
-wash lightly with bestial brown
-touch up lighter areas with a mix of badab black and boltgun metal
-add mithril silver for final highlights
if i don't want such an oxidised/weathered feel like on the socket joints and the back boiler plate, i skip the turquoise and bestial brown washes and go straight to boltgun metal and mithril silver.
Oh and if i want it a bit darker, i shade using just badab black.