I would use a hydrochloric acid based toilet bowl cleaner available at any place listed under "Janitorial Equipment & Supplies" in your yellow pages phone book.

For maintaining a clean appearance, then a weaker phosphoric acid toilet bowl cleaner would be all you'd need.

Neither phosphoric acid nor hydrochloric acid will harm glass or porcelain, even at higher concentrations, so if you have muriatic acid, you could try applying it with a Q-trip or nylon bristle paint brush to see how effective it would be. If it works, then use a gelled hydrochloric acid toilet bowl cleaner which will be about 14% hydrochloric acid instead of 26 percent.