Assuming plaster coving.
Nail wooden battens wrapped in clingfilm under the coving and aligned straight and level. Fill the gaps between with a propietary filler or even Plaster of Paris. Let dry. Remove battens. Make good any small imperfections and fill nail holes. Sand where required. Decorate.
Alternatively remove existing and replace with new coving especially if unevenness is excessive.
Peccavi, October 2008
