It is generally not recommended to put a sheet pan with rust on it in the dishwasher. The combination of water, detergent, and the high heat in the dishwasher can potentially worsen the rusting and corrode the pan further.
Here's what you should do instead:
Remove loose rust: Before cleaning the sheet pan, remove any loose rust using a scrub brush or a scouring pad. This will help prevent the loose rust particles from spreading in the dishwasher.
Hand wash the pan: Fill your sink with warm water and mild dish soap. Use a non-abrasive sponge or dishcloth to scrub the sheet pan thoroughly, paying extra attention to the rusty areas. This should help remove any remaining rust and food residue.
Rinse and dry: After washing, rinse the sheet pan with clean water to remove any soap residue. Then, dry it thoroughly with a towel to prevent moisture from causing further rusting.
Season the pan (if necessary): If the rust has affected the seasoning or non-stick properties of the sheet pan, you may need to re-season it. Apply a thin layer of oil (such as vegetable oil or canola oil) to the entire surface of the pan, including the rusted areas. Heat the pan in the oven at a low temperature (around 300°F or 150°C) for about an hour. This process helps to create a protective layer and restore the pan's non-stick properties.
By hand washing and properly maintaining your sheet pan, you can help prevent further rusting and prolong its usability.