Rinse the flask with hot water and empty the flask.
Squeeze approximately 2 tbsp. of toilet bowl cleaner into the flask, taking care to turn the flask as you pour so that the cleaner runs down the inner sides and coats the entire surface. Allow to soak for 10 minutes.
Fill the flask half full of hot water.
Scrub the inside of the stainless steel vacuum flask, using a long-handled bottle brush and brushing lightly with an up-and-down motion to mix the cleaner with the water and remove any remaining stains from the sides.
Rinse the flask several times until all cleaner residue is gone and the rinse water runs clear. Your flask will now be clean and ready to use.