If you accidentally left your cast iron skillet out in the rain, it's not necessarily ruined, but there are some steps you should take to prevent further damage and restore it:
Dry the skillet thoroughly: Immediately remove the skillet from the rain and dry it thoroughly using a clean cloth or paper towels. Make sure to dry both the interior and exterior surfaces, as well as the handle.
Remove any rust: If you notice any rust spots on the skillet, you'll need to remove them. You can use a gentle abrasive like steel wool or a scrub brush to scrub off the rust. Apply some elbow grease while scrubbing, but be careful not to scrub too vigorously, as you don't want to damage the seasoning layer or the skillet's surface.
Re-season the skillet: After removing any rust, it's important to re-season the cast iron skillet to restore its non-stick properties and protect it from further rusting. Seasoning involves coating the skillet with a thin layer of oil and then heating it to create a polymerized oil surface. You can do this by applying a high-smoke-point oil like vegetable oil or flaxseed oil to the skillet, spreading it evenly, and baking it in the oven at a high temperature for about an hour. This process helps to create a protective layer and improve the skillet's non-stick properties.
Store the skillet properly: Once the skillet is dry and seasoned, store it in a dry place to prevent future exposure to moisture. You can place a paper towel or a cloth inside the skillet to absorb any residual moisture and protect it from dust or other contaminants.
By following these steps, you can often restore a cast iron skillet that has been exposed to rain. However, if the skillet has been severely rusted or damaged, it may require more extensive restoration or professional assistance.