Apart from painting, which is the best part of the job, we also have to attend to what we call preparation!
This can be minor hairline cracks or deep cracks you can put your finger into.
Subsidence is a very common problem, especially on period properties. To repair the cracks, they need to be widened and the crack's progression stopped. This is then followed by adding multiple layers of filler before sanding it all down to create a blemish free surface that is ready for painting.

Hairline cracks or cracks smaller than 3 mm are the easiest to fix and can often be repaired using a simple layer of acrylic decorator caulk.
Below is a ceiling on a Victorian property in Sheen that was next to a railway line and had over the years suffered from subsidence and the constant rattling of dozens of trains passing by each day.
First, the cracks were widened before applying deep gap filler. Some cracks were deep enough to require applying filler four to five times to fill them completely. Once the filler dried, we sanded down the entire ceiling before lining it with Wallrock lining paper.