This one is on my list since some month but I have a lot of trouble with the smooth metal rendering, it's always a huge challenge because you have to simulate the reflections by adding elements that are only the discreet abstraction of the environment, even if this one is simply a black background or united. It's essential to bring metal to life through a multitude of nuances to make it believable, so it's a huge challenge, but I think I'm on the right way !