Wayne Whitney wrote:

I'm working on my 1908 wood lathe and plaster coved ceilings, filling
some cracks and small holes. For most of the coves, I can smooth out
my work by running a taping knife straight up the wall around the cove
and across the ceiling. However, the inside corners are alot more
difficult, as it is basically all curved and there are no straight
lines. Does anyone have any suggestions or advice on how to handle
these areas? How were they done originally?

The original plasterers would have had shaped trowels...

Best is to take a smoothing trowel of sufficient dimension and cut the
appropriate diameter to make one...if it's just a tiny amount of small
cracks, could get by w/ a plastic bowl or other semi-rigid piece...