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Mix warm water, baking soda, and ammonia in a bucket. Dip a nonabrasive sponge in the solution and scrub the tub. Rinse down your tub. Dry your tub.
Fill up a five-gallon (18.92 l) bucket with warm water from your faucet. Then use a measuring cup and add 1/4 cup (45 g) of baking soda and 1/4 cup (45 g) of ammonia into the bucket.  Wear gloves when handling ammonia or other harsh cleaners. Ammonia will remove soap scum while the baking soda will wash away grime without scratching or damaging your cast iron tub. Never mix bleach and ammonia together because it creates a toxic gas. Thoroughly saturate a nonabrasive sponge into the bucket and use it to scrub down the basin of the tub. Work the sponge in small circular motions until all of the grime and soap scum has been removed. Spend more time scrubbing especially dirty areas to fully clean them. Remember to use the solution on the fixtures of the tub, in addition, the tub's basin. Use the faucet or a shower head to rinse down the inside of your tub thoroughly. Remove all of the baking soda and ammonia solution that you just scrubbed into it. If you don't have a showerhead that reaches, you can empty out your bucket and fill it with water to rinse your tub. Use a lint-free cloth to thoroughly dry down your tub. Make sure that you lift up any remaining solution or it can leave a white film on your tub.  This deeper cleaning method should be done once every month. Once the tub is dry, rub your hands along the sides and bottom of the tub, to check for remaining soap scum or grime. If you find any, scrub that part of the tub again. Then, rinse and dry it.