The food-grade stainless steel in PlanetBox is highly rust resistant. If there is a spot on your PlanetBox, it’s probably just a mark left behind from the electropolishing process, which smooths the surface of the metal and makes it shiny. Also, certain detergents and how hard/soft your water is can leave dark spots on your stainless. Sometimes electrolytes left on the surface can turn a rust color, but it’s harmless and can usually be removed by gently scrubbing it off with a scrub brush. This may scratch the stainless, but this is purely cosmetic and is harmless as well. There is no "coating" on our stainless to rub off - its pure stainless steel inside and out.