Worn out metal surfaces in Mudbox

An example of old metal surfaces, sculpted and painted in Mudbox. specular made with photoshop.

You see the main effect is achieved through the specular map which basically masked out the rusty areas on the metal. just take your diffuse and darken the red rusty areas so that they wont display any specular effects. then desaturate and lower its brightness. then save this as your new specular map.
I plan to create a short tutorial on creating forged old metal surfaces. sculpting and texturing.