Even though Himalayan bowls are made of metal and look solid, please remember that they are very fragile! Some antique bowls have such a thin bottom that you can hear a clicking sound or even make a hole if you press it with a finger. Always put the bowl on something soft, such as a rubber shelf liner. If you put a bowl on a wooden surface, check that this surface is totally flat with no nails sticking out or any other firm particles. Always be gentle with your instrument. Don't strike too hard with the firm mallet and even if the mallet is soft, still consider that the wall could be very thin and your approach could be critical for the bowl.
If you decide to clean the bowl, it's up to you which method to use, but please remember that using abrasive materials such as sandpaper can influence the sound of your bowl. The traditional method to clean bronze or brass made bowl is polishing it with a paste made of squeezed lemon with wooden ash. Make this mixture using approximately 50/50 lemon juice with pulp and ash, and polish the bowl using a soft cloth.
Any polishing paste for cleaning jewelry will do the job. Make your preference for a copper polishing paste.
Storing the bowls like Russian dolls (one inside another) can be fatal if there is no soft gasket in between the bowl's surfaces and there is too much weight.
If you store the bowls this way, put bottom of the bowl on something soft and use rubber O-rings or rubber shelf liners in between. Don't put a stack of too many bowls one inside the other.