Quote - Because Luxrender is not 3Delight and they treat polygon collisions in two different ways. If the underlying polygons are close enough Lux might create that kidn of effect. This is, of course, speculation. I need to be able to interract with the scene to be able to give you a real answer. Neverthless, if you are sure that the material is set to Matte, then there can't be reflections. It's either a polygin collision or something else. The point is that it's generally better to solve the issue at the source than to use PS. That's because the issue might re-surface later on and there is an advantage in undersatanding what happened. 

 

PS is your friend....