does not need any treatment it's natural oils make it perfect for the always wet nature of a boat. If your concern is purely cosmetic, there are a few things you can do. I would avoid varnish
. It will be slippery and when it fails, and it will, it may cut your butt when you sit on it. Use a cleaner followed by a brightener. I like Nu-Teak and StarBrite. If the soft grain is raised you'll be well served to sand it flat with a hard block and 180grit paper, then clean, then brighten.
This approach has served me well on customer swim platforms, cockpit
grates, benches and others that did not want to be slippery but wanted some protection. A yearly maintenance
item on exterior, it'll last longer inside without UV influence.