A rust ring can form inside the companion cup due to the moisture caused by wet Jetpower canisters or storing the cup while damp. While the rust ring may be unsightly, it won't affect your cooking system's performance. To remove the rust, gently scrub the cup with a non-metallic scrub pad, such as a Scotch-Brite sponge, using a mixture of baking soda and water. To prevent the rust from returning, store your Jetpower canister upside down in the cup with the black cap on, keeping the canister ring off the damp bottom of the cup. This eliminates the cause of the rust ring problem.