Pretty much what Tonzin said. Since I‘m going for a relatively fast approach on OS so I can get to play it, my worn metal is kept rather simple. I build up from Decayed Metal to Thrash Metal (both Scalecolor), then apply layers of thinned down washes (here Agrax [GW], Gore Grunta Fur [GW] and Earth Green [Scale Artist]. If I feel I’ve overdone that step I bring in some metallics again.