I have seen in kegs and glassware that if PBW is allowed to sit for a long time with exposure to air, a white ring forms. I can't seem to get rid of this. I've tried Bar Keeper's Friend, more PBW, Star-San, Sani-Clean. Any thoughts?
Keep the container filled with water and that won't happen. As the water evaporates, it leaves a mineral deposit behind. Try some vinegar or CLR.