Repairing plaster cracks before painting & decorating
With any plaster repairs, hack away any loose or damaged material with a bolster and hammer, brush away dust, damp broken surfaces and area using a PVA mix with water. Pick up plaster on the underside of the trowel and press into hole, adding more until not quite level with surrounding good plaster. Leave to harden, scratching lines to make key for top layer. This is best put on with flat face of a stripping knife, finishing by smoothing with the knife wet. Plaster will stick to a dry blade.
When the final layer is dry, sand down the wall using sandpaper and use wall filler to get a good finish, before sanding down the excess filler. The wall crack then needs a coat of primer or undercoat before the final painting coats can be applied. It is very probable that the whole wall will need to be repainted in order to match the paint colour, at least professional painters and decorators should opt for that.