Anything you have got to hand really.
Melted Wax, such as Candle fat, Cheese wax (more flexible) etc. Old leftover Paint Dregs, PVA adhesive.
Or if you are flush there are comercial End Seal emulsions specifically aimed at the job.
Anything that can form a moisture resistant coating.
I recently got a big block of paraffin wax from Axminster, pretty reasonably priced. Used a hot air gun to melt shavings direct onto the end grain. Worked really well. Next time I'll be borrowing the cheese grater from the kitchen!