Purge yes, pre-heat no. Practice on a junk first to set amps before tests. 308L, sure it's not 309L? 1/32" lack of penatration is ok, 1/8" to flush on the root reinforcement, 1/16" weld reinforcement, and 1/32" undercut if I am not mistaken. It has been awhile since I have tested myself and I do not have a copy of ASME Section IX in front of me.