I would use either Minwax wipe-on polyurethane (it's oil-based; I usually do 5 or 6 coats), or Varathane water-based spray spar polyurethane. I get both at Menard's. I know Home Depot has the Minwax wipe-on; I don't know if they carry the Varathane spray.
The Varathane is the quickest easiest way I've found to do a refinish. I've done a few longbows and a couple of older semi-recurves with it. I can put on 5 or 6 coats in an hour, and I'm done. Light coats work best. I then let the finish dry/cure for a few days before using the bow. I've only used the satin variety-- it also comes in semi-gloss and gloss.
If you're looking for a smooth glossy finish, I'd use the Minwax wipe-on. It's oil-based, so it will have a slightly amber tone, whereas the Varathane does not.