Concrete back is much smoother, now that I dry it under silicone and the weight. The LED doesn't shine through; I didn't really expect it to, but I wanted to know. Cutting the mold to make room for the legs worked great though. Silicone is pretty magical.
The front is still a little crumbly. I think this is from not letting it sit long enough. It sat overnight, but with the weight on the back, a lot of moisture stayed inside. I'm planning to try different mixes, too, and get it finer and drier. It feels really nice in the hand, though.