How to clean stains from inside of vacuum flask?
I have found that a tablespoon of Oxy Clean followed by boiling water will remove even the harshest stains inside a vacuum bottle. Fill it to the brim (it will bubble and overflow, so do it in a sink), let it sit for a bit, then rinse very very well, and you will be amazed, I promise. I do this periodically to remove the coffee stains inside our steel thermos flask. [Edited for typos]