A bespoke wood window chap came down to Trinity to re-do a few of my grandad’s 100 year old window frames. He had them built with plain, treated wood. I got chatting to him and asked if they mightn’t benefit from a coat of paint. He bent down and picked up a washing line prop off the lawn and asked how old it was.
I hadn’t a clue but it’s older than I am and I told him so. He explained that the reason it’s lasted so long is that it hasn’t a coat of paint for the water to sit behind / underneath – probably as old as the original window frames.
So. When I built my new fence last Autumn, I used treated, plain wood. It looks pretty gash but hasn’t rotted yet. Come back in 80 years and I’ll let you know.