Scrape off loose rust with a metal scraper or grill brush
Wash the griddle with warm water and mild dish soap
Scrub rust spots with a scouring pad, steel wool, or grill stone
Rinse thoroughly and dry completely with paper towels or a clean cloth
Heat the griddle on high until all moisture evaporates
Apply a thin layer of cooking oil to the entire surface
Scrub again with a pumice grill stone or fine abrasive pad if rust remains
Repeat cleaning, drying, and oiling until the surface is smooth
Re-season the griddle by heating it and applying multiple thin coats of oil
Store the griddle covered and keep it dry to prevent future rust