A rust ring can form inside the companion cup if the Jetpower canister is wet or stored while the cup is damp. Though unsightly, this rust ring will not adversely affect your cooking system's performance. To remove the rust, gently clean the cup with a non-metal scrubby, such as a Scotchbrite sponge with a mixture of baking soda and water. To prevent the rust from re-appearing, store your Jetpower canister upside down in the cup, with the black cap on. This keeps the canister ring off the damp bottom of the cup, eliminating the cause of the problem.