I have imported a model as obj. from rhino to blender. When i use the Cycles render, some faces are black no matter which material i choose. I have already tried to flip the normals but that didnt work. I also tried to delete a face when in edit mode and replace it with an extruded edge but that doesnt fix the prblem either. I can only delete the faces and replace them with planes in obj mode. Here are some pictures of the problem. enter image description hereenter image description here

Also in material preview mode some models appear with a gradient on them.enter image description here



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