How do I clean a 50 years old cast iron bath tub?

Hi Frank - I found these directions online:

How to Clean a Cast Iron Tub

• Method 1 Soaking the Tub. Soak the tub in warm water and dish soap for 30 minutes.
• Method 2 Removing Soap Scum and Grime. Mix warm water, baking soda, and ammonia in a bucket.
• Method 3 Maintaining a Clean Tub. Rinse and dry down the interior of your tub after each use

if it is seriously rusted your best option rather than doing further damage to the porcelain surface is to have the tub professionally reglazed. Years of 'scrubbing' with abrasive/caustic cleansers have damaged the surface.

Grandmothers of yesteryear relied upon Bon Ami cleanser, because it does not scratch porcelain covered cast iron.