The endless carport rebuild approaches the point at which it can be roofed. Prior to doing so I have to fit the sides where it extends over garage and rear extension. My plan is to use exterior ply panels. These will be removable once the carport is roofed to allow for easy replacement but I want to increase their life as much as possible. What can I treat the ply with? Any rain which drains off the treated surface feeds a pond in my back garden and which contains newts. Therefore I can't use anything that is hazardous to fish etc. I've looked at Creoseal which (a) doesn't soak into the ply, (b) has a nasty oily run-off and (c) according to the tin is definitely not fish- friendly.
TIA
Richard