Hi Eoin, I would fill in all your gaps and dents first.
For large gaps, shape pieces of planking and glue them in, for other dents use Ronseal wood filler great product it does not shrink and you can sand it easily.
When dry you can sand it using medium then fine, when you are happy with the finish give it a coat of sanding sealer, you also want to tile the Hull.
I hope that helps best of luck with your build

Mike
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