The best way to fix a cast iron bathtub

Clean the  cast iron bathtub's  surface by wiping it down with a rag dampened with acetone.
Remove surface rust spots by scrubbing them away with a mild, abrasive cleaner and steel wool.
Remove penetrating rust by scrubbing it away with a solution of half muriatic acid and half water, using a rag. Always add acid to water--not the other way around. Wear long sleeves, acid-proof gloves and safety glasses. Do not let muriatic acid go down the drain. Sprinkle the treated areas with a generous amount of baking soda to neutralize the acid so it can be rinsed. Ventilate the area for the release of gases.
Fill the nicks with porcelain repair filler, using a putty knife. Allow the putty to cure according to the drying time noted in the manufacturer's instructions.
Sand the patched areas smooth with 220-grit sandpaper. Wipe away the residual sanding dust with a damp rag.
Paint the patched areas with a coat of porcelain primer, using a small paintbrush. This may be referred to as bonding agent.
Paint the patched areas with two to three thin coats of porcelain-epoxy refinishing paint. Allow five minutes of drying time between coats. Allow the final coat to fully dry before using the cast iron bathtub