There is a good chance that there was aluminum used to create that color, I really don't recall that being an original GM color, check the paint code under the hood with a local paint shop to be sure. If it is aluminum in the paint as I suspect, then there is little that can be done to stop it from being further exposed as you continue to polish through layers of paint. You may want to consider not polishing it too much, just enough to bring out a clear smooth finish, claying it, and then putting on some wax or sealant on.