I have washed lots of feedsacks. Those stains look like mold or mildew stains from here. Very hard to get out. I have used just about everything imaginable. But, there is a Resolve two sided bottle.(can't remember the name) but I think it has perioxide in one side that when mixed with the other side and applied to the stains it helps. I think you only are supposed to leave it on for about 4 minutes and then wash. I put my feedsacks inside a pillow case to wash. That helps prevent any fraying or getting threads wrapped all around them during the wash. I have also let them sit for a day in Oxiclean. Good luck with your sacks. They are beautiful!