Filling your stainless steel Thermos with coffee in the morning will keep it hotter longer than a normal coffee mug. However, coffee can stain your stainless steel Thermos and make it look dirty and dingy. A stained Thermos can also affect the taste of fresh coffee that is placed inside the Thermos. Cleaning your Thermos and removing tough stains such as coffee can be accomplished with relative ease. Before beginning the stain removal process, remove any leftover liquid that may be inside the Thermos.
Fill a sink with warm water.
Add a few drops of a mild dishwashing liquid to the warm water. Mix well.
Submerge the stainless steel Thermos into the water. Allow the Thermos to soak for about one hour.
Drain the water from the sink.
Rinse the Thermos in a solution of equal parts warm water and distilled white vinegar.
Towel dry the Thermos.
Mix 1 quart of warm water with 1/4 cup of baking soda.
Fill the stainless steel Thermos with the mixture.
Scrub the inside of the Thermos with a bottle brush.
Allow the baking soda-water mixture to sit inside the Thermos overnight.
Discard the mixture from the Thermos. Rinse with warm water. Towel dry.