[header] How to clean a cast iron tub [title] Soak the tub in warm water and dish soap for 30 minutes. [step] As your bathtub fills with warm water, add a couple of squirts of a mild dish washing liquid to it. Let the mixture bubble up and create suds. 
Once it's filled, let the solution soak for 30 minutes. Leave your tub filled up and let the soap solution break down soap scum and grime.