Coving is a type of wall-to-ceiling transition where, instead of a hard 90-degree angle, the wall and ceiling meet with a concave, rounded corner. Modern coving is made of polystyrene and coated in a hard material, but original coving in older homes is made of plaster. Over the years this plaster coving may have been damaged, and you may wish to repair it.
Pull out any loose pieces of plaster from the hole in your coving.
Insert a small scrap of drywall board into the hole. Fill behind it with some expanding foam insulation, like Great Stuff, available at any hardware store. This will hold the drywall in place and fill the remaining gaps. This foam will expand a lot; so, fill less than you think you need to.
Place metal lath in front of the drywall so your repair plaster will have something to stick to. Do this before the foam cures and the foam will hold the lath in place.
Cut and tear away any foam that protrudes from the hole once it has cured.
Mix your repair plaster in the mixing tray. Make the mixture thick enough that it will not run when placed overhead.
Apply the repair plaster to the hole in the coving. Apply a little at a time, so you can be sure it's getting pushed into the lath. Allow to cure.
Sand the plaster so that it matches the surrounding coving.
Wear proper safety equipment like safety glasses and a dust mask when sanding.