I have water seeping through my cladding (tongue and groove) on one side of my shed. It is not an old shed so looks perfect from the outside (painted with two coats of Sadolin). I don't really want to strip off all the panels, do you think I could put some sheets of marine ply on the outside (it won't be seen from the front) or do you think I should somehow waterproof it from the inside? Any assistance really appreciated, thanks.