Rust is always a pain to remove. But if you have a little bit of lemon juice and salt, you can create a paste that takes it out pretty well. The acidity of the lemon juice reacts with the rust to soften and dissolve deposits.
Fill an oven-safe baking tray with water and lemon juice, and bake it for at least 30 minutes at 250 degrees. After the oven has cooled down, remove the dish and sponge any remaining grime and grease from the oven. Buildup begone!