In reply to mutt:
Well, yeah, plywood does do that when exposed to rain, painting it just slows down the process a bit if you're lucky. If you're putting it outdoors then i'd imagine the best plan is to put a proper roof on it - like a massive shed roof made with something decent, not that bitumen roll stuff that disintegrates after a few years. Would be relatively easy to put over a beehive-kind of wall and not take up any extra space (as opposed to a big open canopy over the traditional one-big-flat-panel design)
What material will stand up to the long-term ravages of UK weather though? And is it cheaper than decent ply with a roof over it? Bearing in mind that you wouldn't want to use a wall that doesn't have a roof anyway if it happens to be raining, or it has rained and the holds got wet due to lack of roof...