You can get polyeurthane black shiplap now like in the picture above (they’ve invented everything). It’s a bit weird because even though it’s made of PU, each plank is slightly distorted at ever so slight differences to give it that old wood look and it’s a bit weird to fit. We’re not keen on using a normal plastic co-extruded corner though. For the cost of the stuff you may as well go back to treated and painted feather edge!!