weather proofing

Our coop is made of plywood and painted with fence preserver though it lets the water in somewhere we have looked but cant find the leak. We had the idea to cover it with pine cladding and painting this with a cresote type of thing as I understand it is difficult to get hold of the real thing. Is this very advisable as I am concerned that red mite may get between the ply and the cladding, ir if we should just start again and make a new upgraded house.