What to waterproof cladding with?

I have been asked to help stop a small leak.
Water appears to be coming in through some vertically mounted wooden cladding that surrounds a large hot air duct from a launderette. I can only think that when the wind is blowing rain against the cladding it is getting throught the joints as it runs down the length of the planks.
Is there some sort of thick waterproof coating that could be applied? I think someone has had a go previously with Aquaseal or similar bituminous roofing sealant but this has gone flaky over time and is certainly not working.
The alternative is to rip it all off and reboard with something, or even think of a solution that would leave it exposed, but protected from mechanical damage from people passing etc.
I await all your wisdom.
Thanks as always. Rob
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it runs up through a flat roof of the launderette then up along the side of the building above (flats above - 2 storeys) to roof level, where it vents.
It has been clad using something that looks like a heavy duty tongue and groove matchboard or similar and the lengths have been run vertically, along the length of the flue. It has subsequently been painted with a black, bituminous type coating which is now flaking.
Although there are no gaps where one can see the actual galvanised metal flue beneath, some large gaps are visible where the tongue joins the groove (good name for a band that!). My thought is that water running down this cladding is, with something akin to capiliary action, being drawn along the grooves and to the inner of the cladding, where it is then running down the inner surface and then through onto the ceiling of the launderette.
Is that giving a better picture? Hope so.
Rob
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