Cleaning a gray cooler is relatively simple. Start by emptying the cooler and removing any remaining food or drinks. Use a mild soap or detergent and warm water to wipe down the interior and exterior of the cooler. For stubborn stains or odors, you can create a mixture of baking soda and water and scrub the affected areas. Rinse the cooler thoroughly with clean water and allow it to air dry before storing or using it again.