Hi diane, I would say not really. Polystyrene can be emulsioned but putting anything else on it can be dangerous, ie fire hazard. A lot of these metalic paints are solvent based and if you tried to put that on the coving, it would just melt. Probably your best bet would be to go to a mixing centre like B&Q and look at all the colours they have and see if there is one that looks similar to what you are after and get it in a silk or eggshell finish so that you would get a similar effect to a metalic finish.
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