Keyshot keep zbrush material groups

#1

Dear All,
I wish to render my scene in Keyshot 5. I wanted to assign different materials to one object of mine, but when I import it to keyshot it doesn’t matter if I tick the “retain materials” and “keep individual parts”, when I assign a material it covers the whole object. Unlink did not seem to work.
I seek advice: How to assign different materials to one object? (Exporting multiple objects is not an option, I want to render a silver object with some oxidized parts on it. The clean silver should be shiny, the oxidized shall be mat/dull. I made a texture paint but it looks a bit silly with shiny oxidized parts.
Thanks in advance.