If it is truly sycamore, one of the maples, this isn't a durable wood for exterior use. Durability in timber technology parlance refers to a wood's ability to cope with ground contact exterior conditions, particularly decay resistance. Sycamore is classified as "perishable" meaning an expected life of five years or less in ground contact, and hard maple from North America is classified as "non-durable", i.e., a lifespan of less than ten years, again in ground contact. You'll get longer lifespan if the wood simply experiences exterior conditions without ground contact.
You'll struggle to keep the wood white and clear with any kind of finish, but your best bet is a high quality yacht, marine or spar varnish such as Epifanes or the International brands. They will yellow the wood because of their linseed oil content, but they are popular with yachty types for a good reason - they're not cheap, therefore they're good. Even so, I suspect trying to keep the wood as good as you possibly can will be a real and ongoing struggle involving rubbing (sanding) back and reapplying the varnish every six months or so, just like the yachting fraternity seem to spend more time keeping up the brightwork than they do actually sailing! Normally this kind of repair work needs doing about every 12 months because most people choose a durable wood for external wooden items, but with sycamore being perishable I strongly suspect your maintenance routine will be an ongoing and twice yearly battle.
Personally, when I make exterior furniture I choose a durable wood like European oak and leave the finished piece unfinished, and just let it age as it feels like. It's my experience that 95% of customers that buy outdoor furniture simply will not do the maintenance necessary to keep it looking good, so it's really just a waste of my time putting a finish on in the first place. Slainte.
