What type of wood is it and what type of finish is on there now?
For straight up water proofing bare wood I like Thompsons water seal, but it's wax based so it's going to prevent most other finishes from sticking over top later on.
On my Ipe deck boards I'm using Australian Timber Oil because it has UV protectiveness, then on the ends of the boards and on any of the knots, to keep them from checking and splitting I put on Anchor Seal, a thick wax based coating.
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