Are you sure that's not rust from the water?
Often we get a bit of rust in the tub from the galvanized pipes.
Even a home run in copper may have galvanized pipe for the tub spout.
Try putting a plastic container over that spot, and see if the rust is dripping down from above.
Just that little spot will cost me $300?. Wow!! I'm just a renter. I showed the landlord, he humped his shoulders and said it could be iron in the water that's causing it. Which means he didn't care I guess. I probley won't be replacing the tub nor reglazing for that price. But thanks so much!
Try putting a plastic container over that spot, and see if the rust is dripping down from above.
It depends on how much rusted is your bathtub. If refreshing it with home remedies will be too hard, and if you live in the Chicago area, I know one reliable company . http://www.aprefinishing.com/. For 3 years my bathtub is in good condition.Search for tub refinishing OR reglaze in your favorite search engine.