Don’t even think of using regular household food stain cleansers (containing chlorine bleach) since they will not work at all on rust stains (e.g., Ajax®).
Here are three letters you need to know to get rid of rust and
mineral deposits: Z-U-D.
ZUD®, (by Reckitt Benckiser, makers of Spray-n-Wash and Brasso) is the 800 pound gorilla of powdered cleansers and is environmentally unfriendly (also comes in a liquid form). Why does it work so well? Because of its composition. It uses oxalic acid and a very effective combination of abrasives made up of very fine ground quartz and pumice. The combination is potent.
Although oxalic acid is a naturally ocurring organic acid, it is considered a poison. I do not recommend using this product if you have a septic system. Killing the bacteria in your septic is not a good idea (please note the understatement...).
To use ZUD you just squirt or sprinkle it in the bowl on the stains and brush using a toilet cleaning brush or plastic pad. Now add more water to the bowl or sink to allow the stained areas to be covered by the ZUD solution. Add more ZUD where you can’t cover with the solution (like high on the bowl). After the solution has set for about 60-90 minutes clean the area with the brush and either flush the toilet clean or rinse away the solution.