I have neglected my potjie a bit, so it got all orange on the inside bottom from rust. I have used it a few times before. I proceeded to scrub it with hot water and soap using steel wool and a hard brush. Got off most of it but there are still a few spots that wont get off. It looks clean but every time I rinse it and scrub some more, the water in the potjie turns dark. Can I use Coke or something, or should I not worry if there isn't real obvious rust left?