Remove any coverings with a screwdriver so that you can access the rust on the aluminum heat duct.
Scrub the rust with a wire-bristle brush. Work until you remove as much of the rust stain as possible.
Scrub fine steel wool over the surface to continue removing the rust stain.
Spray a rust remover over the area if the rust remains visible.
Let the rust remover sit on the area according to the manufacturer's instructions.
Scrub the rust remover and remaining rust with fine steel wool.
Rinse the area with water and repeat Steps 4 through 6 until all rust is gone. Dry the aluminum heat duct completely with a clean cloth.