Metal is prone to rusting. The primary role of car paint is to create a barrier between the car's metal and everything in the atmosphere that comes into contact with your car. Rust begins in little nooks and crannies on a car where the paint has chipped away exposing bare metal. The areas around the window are especially prone to rust because of the water that concentrates around those areas. It's best to remove the rust around the window before it eats through the metal. However, if the metal is badly corroded, the corroded parts need to be replaced.
Tape the surrounding surface around the rusted areas and cover the window with newspaper. This protects the car and window from rust, dust and other debris.
Clean the rusted area with a hand-held grinder and sanding wheel. Grind the surface with the sanding wheel until most of the rust is gone and the surface is smooth and level.
Clean the area with a rust-removing solvent and sandpaper, which removes the last remnants of the rust.
Wipe the area with a clean cloth. If there are no spots or dust on the area, it's ready to be primed and painted. Repeat Step 3 if the cloth has rust dust or residue on it.
Tape around the affected area, and cover the window with newspaper.
Clean the rusted area with a hand-held grinder and belt sander. Grind the area until the area where the rust stops is clearly visible.
Cut out and trim around the rusted area. Cut out all of the rusted metal that's rusted through.
Purchase sheet metal or cut metal from a donor car. Use sheet metal if the area around the window is flat. If the metal around the window is angled, use angled metal from a donor car.
Patch the hole with the new metal. If you're working on a large area, the new metal needs to be welded to the car. Enlist the aid of a friend with welding skills if you have no experience in this area. More minor holes can be repaired without welding. Trim the new piece of metal so that it's slightly smaller than the hole. Apply roofing sealant to the edges of the new metal and to the edges around the hole. Place the metal in place and firmly press the edges together. Allow the repair to bond overnight before priming and painting.