In reply to benrhyd:
Mine is outside, I built it in April 2020. I have a heavy duty tarp (not the woven type) over it, covering the entire back of the wall and then protruding forward by a metre and a half, forming a canopy. The wall itself, I used treated timber for the framing then used yacht varnish on it, and also gave the plywood multiple coats of yacht varnish, with special attention to the edges and the holes. I set the framing itself on a heavy duty nylon sheet, which stops any water from somehow making it under the wall.
It has been spotless, any water that *might* make contact with it just beads off, no mould issues, etc. In fact, the only things I have that have gone mouldy are some volumes I acquired from the Nottingham Depot when they were renovating, those did not fare remotely as well as the wall itself, so when I make some more volumes I'll need to treat them the same way as the wall.
Post edited at 09:59