I haven't been able to get the inside of my teapot clean. It's a Villeroy and Boch china pot. I've tried baking soda, dish detergent and a mild scrubbing from my sponge scrubber. I've put it in the dishwasher. Still, the inside is stained. I've even let it sit overnight with lemon juice, white vinegar and warm water. Noting is removing the stains. Any suggestions as to how to clean this? I was thinking about trying Bartenders Friend, but wasn't sure if that was too harsh.