Will that dissolve the deposits? And won't a boiling bleach solution damage the stainless steel inside?
forgot to mention that part
It should dissolve the milk ring...and it SHOULDN'T hurt the stainless steel. For a quart bottle, I'd use 3-4 tablespoons of bleach with the rest boiling water.
I've bleached stainless thermos bottles for almost 40 years...and never hurt one.
I remember one time, I came home after getting laid off for the winter...and forgot my thermos bottle in my pick up...and it rolled under the seat, where it stayed all winter. When I found it in the spring...the coffee had molded and turned pretty nasty. I soaked it in a bleach water solution for a week...sparkling clean.
I sold it in our yard sale.
(but it sure LOOKED good.)