I am currently in the process of building a hearth and I was going to put a thin layer of sheet metal between two of the layers of durock. How should I do it? I am using thinset and screws on each layer but thinset, from what I read on the bag isn't recommended on steel. Do I not use the thinset in the layer where the steel will be? Or do I go ahead and use it anyway, along with the screws?