Hi @hf,
Thank you for your question about some hairline cracks in your ceiling.
Cracks like this are fairly common with slight settlement and movement of your house. If more cracks were appearing around your house, it could be an indicator of a bigger issue, but in isolation, I don't think they are too concerning. If you started to see more and more cracks, then I would certainly suggest having them assessed by a professional to ensure there is not a bigger issue at play.
If you stand on a stool and press on the ceiling either side of the crack, is there any movement?
Long cracks like this usually occur at the joins between sheets of plasterboard, which can happen when there is slight movement at these joins, possibly from screws or glue letting go.
If there is movement, you should use a stud finder to locate the frame above the plaster, then drive some plasterboard screws into the framing to better lock the plasterboard in place.
Due to the length and size of the cracks, you would be better off using an all-purpose plaster product such as this Gyprock CSR 2.25kg Less Mess Multi-Purpose Joint Compound.
Once you are confident the plasterboard is well supported, put on some gloves, eye protection and long protective clothing so you can use a utility knife to cut a v-groove along the length of the cracks. Clean out any loose material from the cracks and then use a 150mm Stainless Joint Knife to apply a thin coat of your multi-purpose compound to fill the crack flush with the rest of the ceiling.
After allowing this first filler coat to dry, you should then apply mesh tape along the full length of the crack, then apply a second coat of multi-purpose compound over the tape so that the tape is fully covered. You should build up the material over the tape and feather the edges off so that they run seamlessly into the rest of the plaster.
Once dry, sand the plaster smooth with a sanding sponge, brush off any plaster dust, then prime and paint.
This will give you a good, strong repair that, if done well, will be imperceptible to the eye.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
