The Olympic Deck Cleaner product is quite amazing at cleaning them up, and requires very little work. You can pick it up at any Lowe's, I paid $15 for a 2.5 gallon jug of it. Literally all I did was put it in a sprayer, and sprayed it on the log and waited about 10 minutes and rinsed it off with water. I was amazed to see these results. I highly recommend it.
My mentor (and master craftsman) Tim Bullock should get credit for this, he tried just about every cleaner available and this is about the only one that works as advertised. See attached pic for the results. Those are Eastern White Pine logs, they had been sitting out for about 6 months.
Tim has been using this for several years without any adverse effects on the logs themselves.
