finish - Depends on if you want the color to remain as-is, or to
weather to silver/gray, in which case you don't finish it at all.
If you use shellac or poly, any film finish, you'll be sanding and
refinishing every few years and it will show. Even with marine varnish.
If you use water seal or behr or some other oil based pigmented stain,
you'll still be re-doing it every couple years, just won't have to sand
the peeling areas of film finish, and it won't show.
I kinda like that silvery color, considering the labor involved in
re-finishing already assembled furniture.