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For the sawdust & glue solution standard PVA glue is never going to work on holes that deep because it will never set once the surface dries. It's a great & easy trick for shallow holes, dents and scratches because you can match the timber colour instantly but for deep holes you need an epoxy or resin glue that doesn't air-dry. I'd use Cascamite powered resin (aka Polymite) - it mixes with water, sets rock-solid and is a lot easier to use than a two-part epoxy.
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