We have a local manufacturer of garden benches who supplies town councils all over the Uk, and uses iroko wood. I have such a bench, and, when new, gave it a good coat of Ronseal Outdoor Varnish (the one that says it 'outlasts yacht varnish'). I'm totally impressed by this varnish. It dries to give a rubbery feel, that gradually becomes harder over time. The bench has been sitting in the garden exposed to the elements for a full two years, and still looks absolutely brand new. I'd suggest giving any new wood a generous coat of this varnish before attaching it to the metal frame.
(And Keruing is a generic term for several tropical hardwoods, some of which are very difficult to work with, and are top of the list for blunting tools, thus adding to the cost!).