I am suggesting ways for this question from my own experience.
I had lived in places where the water is hard and long use had caused very hard deposits at the bottom and sides of the vessels used. Such salt deposits get harder and harder over time and then it may be very difficult to remove them. The same problem occurs for plastic containers and plastic pipes too.
The first step is not to be idle or not to postpone the cleaning. We have to clean periodically and regularly at short period intervals.
We can know that the stains have started to become harder when the taps or vessels do not get their previous shine on soft washing them with water and rubbing by bare hands.
At that time we may rub the surface with using bath soap or a little of detergent powder. If the stain is very less this will yield result. After that, from next time onwards the same method may be used to remove stains and bring back shine on the steel surface.
If the stain does not vanish fully even after using soap, then we may use a few drops of 'closet cleaner' like Harpic, Sanifresh etc. Just sprinkle a few drops on the surface. Rub it all over the surface with an old tooth brush or some other suitable brush which can give some abrasive movement over the surface, but not causing abrasion marks. Then wash it with normal water. The shine will be back. You can verify by completely drying the surface using a dry old cloth.
The cleaning should be done frequently at intervals depending on the hardness of water and the staining on the surface.
It is always preferable to keep the closet cleaner liquid suitably diluted to avoid wastage, to avoid hard chemical impact on the surfaces and hands.
As mostly the stains are caused by salts of Calcium, any mild acid liquids is enough to remove those stains. In that way even Vinegar also works. If the stains are very little and we are cleaning regularly, then using the lemon peels or leftover from tamarind squeezer are also helpful.
The above methods can be used for removing such stains from kitchen platform, closet, wash basin, etc. The toothbrush can be used to clean the inside of taps as well as the corrugated or angular narrow portions on the tap.