This (un-threaded) hole is covered in mineral deposits, and the 1/2" pipe holds hot heating water. I want to close the hole, but I'm not familiar with the procedure. (I realize I need to clean it first.) What can I use as a plug? Do I have to make something? Is there a copper fitting for this purpose? Should I try just plumber's putty? Will epoxy and something that can screw-in work?