I’ve had great success with a product called Epifanes varnish. This stuff is not cheap and requires that you apply multiple coats. Following their directions, I used it on an outdoor bench made from white oak. After seven years, it still looks like new.
DISCLAIMER: Any posts on WoodworkingWeb.com (WW) are posted by individuals acting in their own right and do not necessarily reflect the views of WW. WW will not be held liable for the actions of any user.