To clean a metal thermos, put vinegar and baking soda in the thermos, and fill it with boiling water. Allow the mixture to cool, then gently scrub the thermos with a bottle brush, rinse with water and allow to dry.Continue Reading
Use 2 tablespoons of white or apple cider vinegar and 1 tablespoon of baking soda for every cup the thermos holds.
Fill the thermos to the top with boiling water. Do not place the cap on the thermos. Allow the mixture to cool for about 10 minutes.
Gently scrub the thermos with a bottle brush, which can be found in the baby department of most chain stores.
Rinse out the thermos, and dry it upside down to make sure no liquid pools in the bottom of the thermos.