Can this paint be used to paint the aluminum threshold of a sliding door that has turned dull and stained?

Yes, Rust-Oleum High Performance Enamel Spray Paint provides a durable protective coating with excellent resistance to abrasion, fading, chipping and dulling. Be sure to remove all dirt, grease, oil, salt and chemical contaminants by washing the surface with a commercial detergent, or another suitable cleaning method before applying this product.