First sand out the spot completely then fill the resultin mild crater with a goos fine grade of polyester body filler. Sand this patch down to match the contours of the tub, using extra care if it involves the actual drain contours. Purchase a small bottle of tub repair enamel (American Standard is a good source)in the closest colour match possible. The "white" of your old claw-footed baby is anything but white! It actually is a shade of gray. Apply many, many thin coats, allowing each to dry before the next. It may not be perfect but this process should give you quite a nice result. I refinish tubs for a living so I know that this will work. Good Luck with your repairs!