hi,
thanks for all the answers. i managed to get the result i wanted using this UV Toolkit plugin.
TIG: yes, i also thought that painting the group will apply the material to both front and back faces. sadly, this wasn't the case. maybe it could have been a problem with the geometry? my model was actually first generated by script in rhinoceros, and then i exported to 3DS to import it into sketchup for VRay rendering. also, this was a group of groups inside a component, so that my have also been the issue π
dsarchs: yes, i read about that, but since i do all my renderings in vray directly from sketchup...