The most correct (but also most flexible) approach si to create both materials , a patina (surface rust), and metal. Because both of these materials behave diametrically different:
1) Patina behaves like non-metal because it's oxidized metal, so it has little reflection compared to metal. IOR of 1.33- 1.52 is good, the specular color is white. Very rough surface equals to low glossiness values ( <0.2-0.3). The color comes from Diffuse texture.
2) Metal doesn't have diffuse color, the color comes from specular reflection. Simplified IOR can be substituted by using higher numbers, like 4- 40, or using ComplexFresnel IOR by using Siger's plugin.
To mix them use a mask, either procedural (noise places in fallof or AO) or textured. While oxidation process can be gradual, use mostly black and white values, the surface either is, or isn't metallic, the difference is thickness of patina/rust, and such detail should be in diffuse of Patina material.
