Shou sugi ban; burnt surface weatherproofing is an old technique with possibilities. The surface is burnt and then charing is mostly brushed or scraped away leaving a more porous surface which is then simply oiled well. It looks similar to the black/brown of ties sometimes.
What did they use on telephone poles back in the day?
I know an arsenic solution was involved in a lot of treated lumber. (and two folks hospitalized after burning treated pallet wood in campfires and breathing it)
