Bit of a throwback to when I started repainting my house. Normally is a cas eof a quick wall clean, touch up so chips/cracks and get painting. But not in the case of 90% of my walls in my house that are 80% covered in fracture lines. Lines too fine to be filled with standard poly filla and so fine and plenty it takes SO **bleep** LONG TO DO!
Tip: Painting over them or "filling them with paint" will not fix your problem if you want a beautiful finish. It'll hide a few but many will still remain and even the hidden ones will likely show their ugly cracks again soon after.
There is however a great product to solve the issue. Like I said, it's time consuming, takes a toll on your wrist and patience and obviously means the whole walls need to be sanded before being painted -- good times!
Gyprock CSR Final Finish is what you're after and a 50mm spatula. Works a treat. Only applya little at a time to keep the mix "wet" on the wall for easy filling/spreading. Loading it up overworks it, hardens it and makes it unnecessarily more difficult to use. Enjoy!