I inherited a stainless steel instant water boiler kettle thing from the previous tenant (yay!). However, he left a small amount of water in there for God only knows how long (at least two months), and there's a mystery white substance on the bottom of the pot. I scrubbed and soaked with dish soap but can't seem to get it all off. Any tips for cleaning stainless steel? I would love to avoid having to buy a water boiler if I can.