That's a "one-pack" filler. What you need is a two pack filler where the hardener needs to be moved with the filler. Far harder (once set), more durable and waterproof. If not visible (it is on the underside, after all) even P40 car body filler would be good
Errr, no. A 2-pack filler generally comes in a wide mouth resealable can (not a plastic container) with a tube of hardener. The hardener needs to be thoroughly mixed with the filler before applying it to a clean, dry surface. A good example is Everbuild High Performance 2-Part Wood Filler from Toolstation. As stated David's Isopon P40 2-psck filler, available from car places such as Halfords, is another example. Wood not dry? Treat it with wood hardener first
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