Ferrous sulphate (FeSO4) is the common chemical for treating raw wood on buildings - an effective preservative as well as giving the wood a beautiful silvery grey tone. I have not tried it on a kayak, so I do not know if there are any side effects in combination with epoxy. I would do a test panel first.

If you want to try, mix approx 70 gr FeSO4 to 1 litre water, and a couple of drops dishwashing liquid to improve penetration. Brush the solution on to the hull after sanding, maybe twice (let dry between applications) to get a deeper tone, dry overnight and fine sand to cut back the raised fibres, and then laminate fiberglass/epoxy as usual from there.

Björn