"White metal" can mean several things. It used to be a euphemism for lead, or lead alloy. Lead is one of the most notoriously unstable of materials; it frequently degenerates into a white powder (aka "lead disease"). And sometimes it doesn't. I've got some old Model Shipways "white metal" fittings in my workshop that I've had for twenty or thirty years, and they look as good as new. I've also seen lead disease develop in less than a year.
Nowadays, due partly to government regulations, scarcely any hobby products have lead in them. (Good riddance, as far as I'm concerned.) I think what the manufacturers call "white metal" now is usually either britannia metal (a mixture of tin and copper) or pewter (which the dictionary in front of me describes as "any of numerous silver-gray alloys of in with various amounts of antimony, copper, and sometimes lead..." - but not lead any more, as I understand it).
Anyhow, there are various ways to clean it. Several companies (e.g., Bluejacket) sell chemicals specifically for the purpose; they smell so much like vinegar that I suspect that's about what they are (and vinegar is an old-time remedy for the problem). I like to rub larger parts with extra-fine steel wool, which will give it a polish almost like silver. If the part has flash on it, you can trim it off with a sharp knife - as though it were plastic.
One big virtue of modern "white metal" is its flexibility. In most cases, if you do accidentally bend it you can bend it back.
Hope that helps a little. Good luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.