Trying to make a rusted metal material

[Astralogic]

Hello, could someone give a tip or two on how to make a rusty metal material please, I already know the basics of how the material will be set up I just don't quite know how to achieve it.

 

I want to basically create a multi/sub material in which I have (from the autodesk library) four mental ray materials. Weathered metal, light rust, medium rust, heavy rust. All I need to know is how to use a noise map as a transparency mask so I can get a nice mixture of all those maps.

 

Any and all help is appreciated, thanks.

[Astralogic]

Never mind I quickly discovered that sub/multi was the wrong thing to use. Blend was what I needed.