I have a friend that has a 40 year old ceiling that has cracked and the stipple has separated from the plaster. Can I inject some glue behind the flap of plaster, heat it with a hair dryer and gently push the flap back into place ? I have a syringe and needles from Lee Valley Tools. Which kind of glue should I use ? The stipple is the earlier type which is probably just ridges of plaster. I am not sure if the "flap" is stipple or layers of paint. If the above doesn't work and the ”flap” falls off I will have to repair and paint the whole ceiling.
Inject Weld-bond All Purpose Wood Adhesive using a syringe. Compress the surface until an even coat of Weld-bond is on both surfaces ceiling and flap (as you call it). Hold in place with push pins (thumb tacks). Leave overnight until cured.