OK..sorry ......new cornice....make sure thats its dry throughout, it can take weeks.....then first coat it with thinned matt emulsion....say 30/40% water to paint....the second one can be thinned to for ease of application.
I'd go for a matt finish as opposed to silk...which may make it look like plastic...Eggshell finish isnt bad either but again keep to a water based system
Id go for matt simply becuase it emulates the surface of plaster
Assuming your are talking about fibreous plaster cornice as opposed to coving...which should be painted as normal plaster board
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