I built a garden bench a couple of years ago (white oak), and I live in a rainy climate, so it immediately started to turn black and unsightly, but I had to wait until the following summer to do anything about it. The next summer, I decided to sand off the blemishes and finish it with an outdoor varnish; however, this seems to had little to no effect- one year later, and the bench looks exactly the same as it did last time, with black patches and weathering. Is there any product out there which will protect my bench from weathering for at least a couple of years at a time?