There is nothing you can put on the outside that will guarantee a sucess. You basicly have to pull it apart again. I would suggest, however to use teflon tape on the threads and a little teflon pipe dope over the teflon tape. Most of the time what happens is the teflon tape does not seal because it bunches up or slides around. The teflon pipe dope helps allow it to slide inside the threads of the valve body.