I've got an old military sporran that was originally brown, but has been bulled to a deep, shiny black. The sporran is in good shape, but I'd like to touch up the finish and prevent the leather from drying out.

There's not much info on bulling online. Does anybody here have experience bulling leather? If so, what would you recommend? I don't want to screw up the hours of work that have already been put into this finish. Thanks!