The body of the flask is fabricated both inside and outside in quality stainless steel. This makes it hundred percent rust free. It is advisable to pre-condition the flask depending on what you want to store in it. So you can pour hot water if you use it for hot beverages and cold water for cold drinks. However, it is not suitable for carbonated drinks as they may wear out the inner body of the flask. Thermosteel flask is easy to clean. Use a mild detergent or dish wash liquid and clean on the inside with a bottle brush. Do not use very rough cleaning sponges or bleaches, as they may damage the steel exterior of the flask by leaving scratch marks. Also, keep the bottle open when not in use to avoid any odour.