Water collects minerals as it flows. Depending on the rocks and minerals in your area, you may have hard water or soft water. Soft water means that the water has not picked up many impurities, whereas hard water has collected more minerals. Hard water eventually leaves a white, crusty deposit in certain items you use frequently. These deposits are called "limescale." When limescale heats and cools, it becomes harder and more difficult to remove. Cleaning your stainless steel teapot well after using it and regularly descaling it can keep your teapot functional for many years to come.
Fill your stainless steel teapot 1/2 to 2/3 full with white vinegar. If it has been a while since you descaled your teapot, fill it closer to 2/3. Add enough water to fill the teapot.
Leave the vinegar and water mixture in your teapot overnight to soak.
Spill out the vinegar and water from your teapot. Use an old toothbrush to scrub away the limescale.
Rinse your teapot well, and look inside to see if the limescale is completely gone. If necessary, use a flashlight or feel the inside of the teapot with your fingers.
Repeat the vinegar and water treatment if you have particularly stubborn limescale. Once you have established a routine for descaling your teapot, you will only have to allow it to soak once.
Fill your teapot with boiling water for a final rinse to remove the vinegar completely.
You can also use lemon juice if your teapot does not have heavy limescale deposits. For really stubborn limescale, you can heat the vinegar before adding it to your stainless steel teapot.
Avoid using highly abrasive cleansers, steel wool or wire brushes to clean your stainless steel teapot as they will damage the finish.