You all keep talking about hydrofloric acid which are considered relatively weak acids. Hydrochloric acids, on the othe hand, are a different story (also known as muriatic acid). They are very strong and should be used in a ventilated area. They are good for cleaning mineral deposits and uric acid build-up off of old vitreous china toilet bowels and tanks and make them run like new again.
I only put the bowel and tank minus the metal parts on a piece of plywood for cleaning. The acid will eat through gravel, concrete and will kill grass. Use a toilet swab and gloves, hold your breath while applying and run away tell the fumes die down, then go back for more applications. After the parts have been cleaned, especially inside the rim and the trap, you can spray everything down with water. Anyone who's restored an old toilet already knows this.
I'd never use this type of acid on porcelain, a sink or tub. It will ruin it.