There is nothing quite as welcoming as a pristine bathroom. Likewise, there is nothing quite as revolting as a stained toilet. While the toilet may have been cleaned and sanitised earlier in the day, the stains scream filthy and neglected. There are dozens of products in any store's cleaning products aisle, all touting the ability to remove any stain. Some do an excellent job removing stains, but there are other promising options you can find at hardware stores or outlets that sell pool cleaning supplies.
Determine what type of stain is in your toilet bowl. If there is a raised white ring deposited around the water line of the bowl or under the inner lip of the bowl in the openings where the water enters to fill the bowl, the culprit is most likely an overabundance of lime or calcium in the water. If the stains are rust-coloured, they are probably caused by a high level of iron in the water.
Put on rubber gloves, remove any in-tank cleaner cake from the tank and discard it. Flush the toilet several times to remove all traces of chlorine bleach from the bowl.
Insert the plunger into the toilet bowl and press down quickly and firmly several times to force the water out of the bowl.
For hardened grey or white lime and calcium deposits, thoroughly wet a Pummie (a pumice stone with a plastic handle--available at most hardware stores). Rub the stone on the deposits. This will require a great deal of elbow grease and patience, but it will eventually remove the deposits.
Make a paste of Barkeeper's Helper with some water and, using a rag, dab some of the paste on lime/calcium deposits that are under the inner lip of the bowl where the water comes in to fill the bowl. Let the paste sit for 10 minutes or so before scrubbing the deposits with the rag.
For rust stains, after plunging the water down the trap, wear rubber gloves and apply some muriatic acid (available at hardware stores or pool cleaning supply outlets) to the stains. Allow the acid to sit on the stains for 1 minute or so, then use the plastic toilet bowl brush to scrub the stains away. Flush the toilet several times to remove all traces of the acid. If you cannot locate muriatic acid, use a product that has diluted hydrochloric acid as its active ingredient--products such as The Works Toilet Bowl Cleaner and Santeen De-Limer and Toilet Bowl Cleaner. Stubborn rust stains may require more than one treatment.
Avoid trying to bleach stains, as bleach will only set the stains and make them more difficult to remove.
Always wet a pumice stone thoroughly before rubbing it across stains in a toilet bowl to avoid scratching the porcelain.
Make sure that there is no trace of bleach in the toilet bowl when using any product that contains an acid, as chlorine bleach plus acid yields toxic (and possibly deadly) fumes.