Repairing the Finish on a Porcelain Bathtub?

Two spots have appeared in my bathtub again where the white porcelain finish is gone. I previously used a product I bought at either Home Depot or Lowe's (can't remember which one!). While it was gloppy and I was not able to paint it on smoothly, it did cover the spots for about 2 years. The repair finish came off and now I'm stuck with the two spots again.

Do any ThriftyFun members have experience with a product that works?

Thanks very much!

By Caseye