House is over 30 years old. Have original built-in enamel over cast iron bath tub in master bath. Just noticed today that area at 3:00 position around overflow drain is starting to rust through. While I have replaced toilets, garbage disposals, retiled the tub & shower area, and other similarly taxing DIY projects, am not sure how to proceed with this problem. Can the rusted area be fixed so as not to leak? Or are more drastic measures needed? Am planning on retiring in this house, so any fix must be good & permanent.