Don’t panic when it comes to fixing a scratched bathtub. Before crying and telling yourself it’s time to replace the entire bathtub try fixing it first. When replacing a bathtub damage to the walls and floor are common and don’t be surprised when it happens. Be prepared for the worst. Make sure you budget right when it comes to replacing a bathtub. Hope the link below helps, if not ask friends and family if they ever have come across this problem. Make sure to call a plumber when taking out a bathtub. Making sure all the plumbing is in order and not damaged during the demolition.
http://voices.yahoo.com/how-repair-scratch-cast-iron-6923904.html