its probably on its way out
i have used ronsons neutral/pine filler before now but what ever you use will show just make sure you have at least 3mm depth to fill
this one you can get it in the sheds or diy stores http://www.ronseal.co.uk/products/multi-purpose-wood-filler
if you look very closely are the edges between boards raised
has this area ever got wet for more than a few seconds ??
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