Question on using materials in zbrush/keyshot for single object.

[ghostweeble]

Hello,
I'm using Dynamic Subdiv in Zbrush and Keyshot, and I'm having some difficulty in finding a way to cleanly assign a material to a polygroup.

When I fill 2 separate polygroups of the same object with different materials (by hiding one polygroup and filling it with "Fill MRGB") I get bleed into the other polygroup that is hidden, rather than the material just filling the visible polygroup.

Please see the attached visual.

Am I missing something? Is there anyway to avoid this? Basically I just want to "tag" some polygons in zbrush so that in keyshot, I can assign a material to these polys and the lines are clean.

Is there any other workflow that would allow this?

Thanks for your help!