But once you get a build of protective varnish on, you'll notice the dark stains(water stains) less. Some have success with oxalic acid bleaching. Don't try to sand them out, you'll just create a divot in the wood that looks worse.

One trick if you're under the gun for weather Gene, use a fast drying varnish for your first coats. I've done that with good results. You can put 2 a day on(thinning the first couple), don't even sand between, and 3rd next am, and be onto a fourth varnish, unthinned, the next afternoon.