There are many ways to get rid of rust which use everyday household products that you probably already have. To begin with, you’ll need a scrub brush, rubber gloves to protect your hands and a spray bottle. You can use products like bleach, borax, vinegar, hydrogen peroxide, baking soda, etc. Spray or apply it to the rust. Wait for 10-15 minutes and scrub appropriately. You can Scrape off the corrosion on the window case with a wire brush. However, if the rust is spread over a larger area, it's always better to call in a professional.