Depending on how large the area is that needs to be patched you can simply patch it with spackle. If the area is too large, you can get a mesh tape for drywall to help.
My ceilings are uneven and saging a bit in between the batons I suspect that the builders did not put in enough batons now the ceiling is wavy . I want to correct thi...
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can you post a picture of the damaged area?
Depending on how large the area is that needs to be patched you can simply patch it with spackle. If the area is too large, you can get a mesh tape for drywall to help.
Holes? Cracks? Need more information.
Here is a great video from This Old House. https://www.youtube.com/watch?v=Btkt0F7O-O8