On Mon, 14 Jan 2008 15:55:36 -0800 (PST), jim
wrote:

Son - in - law (ex to be) decided to do this old guy a favor and
drywall the cottage mud room. Left the plastering / mud to be. Only
problem is that in several joins he did NOT butt the tapered edges
together. Thus there is no "groove" in which to fill with mud and
feather out for the smooth finish. Any way to remediate this other
than rip it out?

Yep
Tear down the house and build a new one !!!!