Hello. Tannin buildup from tea and coffee can be surprisingly resilient against standard dish soap, but you can easily lift it using pure bicarbonate of soda and boiling water without causing any harm to your stainless steel interior.
The most effective and gentle method requires two tablespoons of bicarbonate of soda for a standard 500 ml to 750 ml thermos flask. First, add the dry bicarbonate of soda directly into the empty flask. Next, slowly pour freshly boiled water into the flask until it is filled to just below the brim. The mixture will froth slightly on contact, so it is best to do this inside your sink.
Do not seal or screw the lid onto the thermos during this process. The heat and slight effervescence can create pressure buildup inside the vessel, which can warp the silicone gasket or make opening it hazardous. Leaving the lid off also protects the rubber seal from prolonged exposure to extreme heat and potential lingering odors.
Allow the solution to soak undisturbed for at least one to two hours. For very severe or long-standing stains, leaving it to soak overnight is completely safe for food-grade stainless steel.
Once the soaking time has elapsed, pour out approximately half of the liquid and use your bottle brush to gently scrub the interior walls and base. The loosened residue will lift away with minimal effort. Finish by emptying the remaining liquid, rinsing the flask thoroughly with clean warm tap water, and leaving it inverted to air-dry completely.
If you ever wish to clean the lid and removable silicone seal, simply detach the gasket and soak both parts separately in a small bowl of warm water with one teaspoon of mild dish soap for fifteen minutes before rinsing.
