Great you found out how to do it even when there's no texture image for the mirror part!
By the way, you might have encountered a bug during the last step that prevented the reimported OBJ model to appear correctly in the 3D view. I fixed this bug, but until the correction is released, the workaround is to reimport a ZIP file containing the OBJ file, the MTL file and its images, instead of reimporting the OBJ file itself.
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Emmanuel Puybaret, Sweet Home 3D creator
