The ebony stain would be fine. Just protect it with an out door finish on top. The polyurethanes are OK if they are specifically for out doors. A film finish (not a penetrating oil) needs to be flexable enough to expand and contract with the wood movement. A spar varnish will do that, but will eventually crack and will need to be sanded to refinish. A penetrating oil just needs another wipe-on coat to bring it back. Sun (UV) and wood movement (expansion and contraction) are the enemies of finishes in the out of doors. I have used deck stains, varnish, polyurethanes, and oils for my out door furniture and like the oils best. Tung oil, lindseed oil, and my favorite Mc Clusky's out door oil, are easy to apply, easy to refinish and look good. The more coats the more shine. They are soft so you'll have to recoat more often due to scratches but it's easy to do.
John
