The sink is starting to get porous on the bottom from all the harsh chemicals that have been used on it. A mixture of half water and half Clorox has been used to bleach it back white, but this is very toxic to breath.
Is there a different product you know of that will work on this? Also, can the sink be polished with something to help prevent the staining?
Contact a bathtub refinisher in your area. These folks specialize in resurfacing fixtures such as yours.
This might be a simple fix for a reasonable cost to your customer. If it proves effective and lasting, you may have ... — Lynn E. Krafft, ICAN/ATEX editor