If you brush the powder onto the mold, it gives a nice realistic shiny metallic finish. here's an example of the process: http://www.youtube.com/watch?v=biYUuA100EE
This one is using the "gun metal" powder. The "silver" powder (which is actually aluminum) is brighter.
Paradoxically, clearcoats tend to reduce the metallic effect; they make it look more artificial.
I can't say how much Rub'n'Buff weakens the latex. I'd do a test, but I think my latex is expired. If dead set on using rub'n'buff, and it does have a significant effect, you might be able to protect the latex by first rubbing in a thin layer of castor oil. You can get castor oil at your local pharmacy. If they don't have it in stock, they can usually order it and have it in one business day.