When your fence is discoloured you should look at replacing your fence. You can treat it but it won't go back to its original state. If your fence has started rotting it's time to get rid of any rotten material and replace it with new and durable panels. If you are completely re-doing your garden then you should replace your old fence with new fencing. It will look more in style with your garden and that way you won't have to dig up any part of your garden down the line. You can do it all at once.