You can use cutting oil, penetrating oil, lime remover (if there is extreme lime or rust buildup), WD-40, or even vinegar for this process. Otherwise, you can fill a plastic bag with vinegar, wrap it around the showerhead, and let it sit for 2 hours to dissolve rust. If you fixed the issue, caulk around the baseplate to seal the unit to the wall, letting it dry for 12 to 24 hours.