CLR, a commercial liquid cleaner, mentioned above, is widely available. It acts quickly and very little is needed. Initial cost seems high, but it goes a long way. I use it to clean my taps and sinks (water extremely hard), and the quart container must be two years old. Never use abrasives on porcelain (also damages aluminum sinks, etc., etc.).

The chemical action of phosphoric acid is different from that of hydrochloric; we used both in a lab I was connected with (although some uses overlap).

Several cleaners have phosphoric acid content; makes me wonder if phosphoric-containing fertilizers might work! On the other hand, phosphates may be too stable for quick action. Never thought to try it. There are worries, however, about cumulative effects. Phosphoric acid in run-off causes plant blooms in streams and lakes, and damages the environment.