If you're using anything abrasive, (T-cut, Autoglym, chrome cleaner etc) it's a good idea to use masking tape around the brightwork because you can easily rub through the paint, which is less than desirable...
If the brightwork is anodised aluminium, once it starts going milky, it's the begimming of the end.. You should be able to get it shiny again with abrasive cleaners, but you'll never again be able to leave it out in the rain for say 5 years without re-polishing.
The anodised sill finishers on my Allroad went that way so I just sprayed them with bright silver paint. Looked almost exactly the same as new.