If you drill out the center of the old zinc ti say 1/4", you can use an easy out to unscrew it from the brass plug. I clean the threads with a small sort of bottle brush with metal tines I put it in an electric drill and clean out the threads that way. They come out shiney and clean. No acid needed. I do put a dab of Koppershield or silicon grease on the threads when I assemble it. That seems to make the zinc unscrew from the brass more easily. Cheers, Tom 410.212.7898