All MontiiCo products are made from 18/8 food-grade stainless steel. This food-grade stainless steel is highly rust resistant, as well as safe, durable and BPA-free. Sometimes marks can be left behind from the electropolishing process which smooths the surface of the metal as electrolytes left on the surface can turn a rust colour, but this is perfectly normal and harmless.
To remove residue, we recommend that you leave your bottle/cup/food jar to soak overnight with hot water and baking soda or white vinegar. After soaking, give your bottle/cup/food jar a really good scrub using a bottle brush or a scouring pad. Let us know how you get on after trying this method!