We are wel under way and are ready for tile and finish. We have decided to do black galaxy pebble fina. We are considering black gauged slate for the waterline and exterior of spa to match our patio already installed. Does anyone have any experience or advice on this topic? I wasn’t sure if it had to be specifically designed for waterline or if any would work. It says suitable for indoor outdoor freezing conditions. Below is a pic of the slate we used for patio. We have not seen any considerable flaking with the patio. Any help is much appreciated!!