ZBrushCentral

Zbrush to Keyshot 5 different materials to one object

Dear All,
I made a silver object and I applied some polypaint to imitate some oxidizing on it.
I did that with different materials. When I import the obj into Keyshot 5, it doesn’t matter if I applied different materials back in Zbrush and ticked both “Keep individual parts” and “Retain materials” (group by materials) in Keyshot when importing, the time I assign a material to my object, it covers the whole of it. It is not an option to export multiple objects - due to the fact that a spray polypaint is the oxidized part - and it looks quite silly with shiny oxidized layer instead of the mat/dull one.
So again: different materials to one object in Keyshot 5? I tried the unlink trick, but it did not work, I might did something wrong.
Thanks in advance.