You Polyfilla or Spackle! Use a scraper to push it into the crack and smooth over. Rub down when fully dry.
If it is a hair line crack simply fill it with some spackle then after it dries sand it smooth and paint over it.
Apply spackle/ dry wall mud to fill the cracks, let it dry. Sand it smooth & retouch it w/paint to match.
Compound put on with a putty knife as flat as you can when dry sand flat then paint
thanks for your help