Mmmm... The flecks and rays in Sycamore are beautiful. I'd think you'd have a higher use for sycamore. With that said I recently salvaged a bunch of barn rafters that are sycamore. I'm thinking of using them for interior window trim. Would they work on the exterior? Probably. I'm using white pine for B&B, and I don't see why sycamore wouldn't work. Any wood will work as long as it can dry out and does stay wet. Pine and oak barn siding have lasted 100+ years so why not. Just keep it away from the ground. The bottom of the old barn siding used to rot because dirt would splash up and they would dry out at the bottoms as well. I'm doing an 18 inch veneer stone at the bottom which will get my siding off the ground.