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Run down to your local pharmacy and pick up a pack of dissolving denture cleaning tablets. Most denture tablets contain sodium bicarbonate (baking soda) as an active ingredient, which fizzes when the tablets are added to water. They are designed to eliminate stains from artificial teeth as they soak, which means they’re perfectly safe to use for objects that you’ll be putting in or around your mouth.  A pack of denture tablets will only run you a few dollars, providing many uses in cleaning your well-used flask. The dissolving action of denture tablets also has an antibacterial effect, sterilizing the flask as they clean. Fill your empty flask to about the halfway point with warm or hot water. The higher the temperature of the water, the faster it will begin to loosen the hold of troublesome stains. Allow the flask to soak for a few minutes before moving on to the next step. Swish the water around inside the flask to make sure it’s completely wet throughout. This will help the sodium bicarbonate react on all parts of the flask’s inner surface. Drop a couple of denture tablets into your water-filled flask. The chemical reaction will cause the water to bubble and foam up, so it’s best to do this in the sink, outside or somewhere you don’t have to worry about making a mess. Don’t cap the flask—this will cause pressure to build up inside. A good general guideline is to use one tablet for every two cups of the flask’s volume. Walk away from the flask while the tablets do their thing. As they dissolve, the effervescent action will also disintegrate the buildup on the walls of the flask. Allow the flask to sit for up to half an hour, until the effect of the tablets has begun to diminish.  Denture cleaning tablets are one of the safest and most effective ways to clean a dirty flask or thermos. All you have to do is wait. Once the reaction has died down, you can go over the inside of the flask with a bottle brush for a more thorough clean. Pour out the water that the denture tablets dissolved in. Run fresh water in and out of the flask a few times to clear away whatever traces are left behind. While the flask is drying, keep it right-side up with the cap off. Afterwards, it will look as good as new! Putting the cap on your flask while it’s still wet will encourage bacteria to set up shop inside.

Summary:
Buy a package of denture cleaning tablets. Fill your flask with water. Add one or two denture cleaning tablets. Let the flask sit for awhile. Rinse repeatedly.