If you have a porcelain bathtub or sink, you know how dingy they can get over time. You can keep them looking their best with a simple mixture of baking soda and ammonia.
Cast iron fixtures typically have a baked-on porcelain enamel. You can't use abrasive cleaning products because they can damage the finish. The folks over at Apartment Therapy recommend weekly cleaning with hot water and a bit of dishwashing soap, but for occasional deep cleaning, they turn to baking soda and ammonia. Just mix 1/4 cup of ammonia and 1/4 cup of baking soda into a paste. Scrub the porcelain with a non-abrasive sponge and rinse well. For extra grimy fixtures, you can also let the paste sit for a while before scrubbing. After rinsing, use a soft cloth to wipe everything down so the baking soda doesn't leave a film.
How To Clean an Old Porcelain Enamel Bathtub or Sink [Apartment Therapy]