Pour two cups of hydrogen peroxide, four tablespoons of white vinegar, and one-and-a-half teaspoons of table salt into a plastic spray bottle. Vigorously swirl the bottle to mix the contents. Once the salt has dissolved, spray the solution over the object to coat it partially or completely, depending on the desired effect. The peroxide should begin to bubble on contact with the metal, and rust will start forming immediately. Let the object air-dry in the sun for another five minutes or longer, depending on the size of the object.

Thanks, Bob Villa. If you have time to prepare the surface, a volatile degreaser like acetone will remove any oils that might protect the metal and evaporate pretty quickly.

HTH (pool chemical) is also a strong oxidizer. I’m not sure how to best use it, though