Well...it's hard to say w/o seeing it
I'd say it's probably not necessary, so to speak
But it couldn't hurt
Honestly, often in cases like these I will simply prime the whole thing just to be sure
(...my loss if I have to fix it later..and we know how time/material consuming that is)
It may be overkill, but I figure a little overkill is well worth it for the No Worries Factor
Dig. I would rather an extra hour or two of priming than losing three or four days scraping and waiting for joint compound and various coats of primer/paint to dry. For the $50 it may cost... it is well worth belt and suspenders here me thinks.
Ok, a month later and having switched to different primer, everything is super. I am a believer in that Gardz stuff. Holy crap does it work well... again, awful to work with, but it creates a great solid surface. PVA never again.