Rake out the crack wet it back with brush and plain water, fill slightly proud with good quality filler from any DIY outlet allow to thoroughly dry sand it down flat with surround and then re-paint it. For wider and deeper cracks add joint mesh [Sold by the roll] used on plasterboard joints. Rake hole/crack out dip the mesh in filler made wetter than that used for normal filling so it impregnates the mesh. Place the mesh in the crack pushing it well in, leave for about 1/2hr the final coat with a standard stiff creamy consistancy, again fill proud wait for 2-3 days for the lot to dry before sanding try too early and it will pull in out if it hits a strand of the mesh.
Larger areas the trick is to prevent fast suction of water from plaster causing crazy paving finish. Mix one part Unibond to 5 parts water and liberally coat the repair area. Allow to dry. Re-coat with approx 1 part Uni to 3 parts water allow to become tacky then plaster the repair or 'Easy plaster' from DIY stores it takes longer to set Only skim for a finish around 1/16 -1/8th ins or 1-2mm. so build up deep gaps in layers.
Bob, December 2008
