We have a ceiling with a smooth emulsion finish, unfortunately long
cracks appear in the course of every few years - one along the line of a
plasterboard joint and two others that are not in line with the
plasterboards.

I have repaired them with a d-i-y filler (made up polyfilla or similar)
in the past but have decided to do something that will hopefully be a
bit more permanent this time.

I have raked the cracks out - the depth appears to be around 10mm and
was wondering about applying a gap filling adhesive (No more nails?) in
the apex of the crack and finishing it off with either a light weight
sandable filler which I imagine will give the finish most likely to
blend in once painted or possibly caulk which I understand has the
benefit of being slightly flexible but may not blend in as well as the
sandable filler.

I would be grateful for any suggestions or comments.

--
Robert