Golly... I spent hours over the course of several days, trying 3 dozen variations, to track down specifically what was causing it. Results are frequently hard to duplicate. Sometimes it works, sometimes it doesn't. The order which you create the various comparison props seems to have something to do with it. I came up with a solution at least twice... then later found out my solution wasn't working. The best I can do is add a BLACK DISPLACEMENT item to the one-sided square in the materials room... and I've had no problems since. If the see-through effect occurs... you'll probally NEVER get rid of it! You have to delete the square and start anew. I am not about to spend another few hours generating various examples for you. If interested, investigate it yourself. Sorry Bill