On a recent Friday afternoon I was called away from my office on an emergency, leaving behind my thermos mostly full of coffee. When I arrived back at my office on Monday and emptied out the thermos bottle, I discovered that the bottom was covered in little black squiggly marks.
I tried washing it out using a narrow brush, but the marks remain. Is my thermos permanently stained? Or worse, was the steel pitted by the acidic coffee? Is that possible?
Thanks.