I am not at all sure why you want to cover the air vent in your chimney breast?
The idea of an air vent, is to help keep the chimney dry - blocking the vent isn't a good idea.
(Is the top of the chimney weather tight?)
If you must block it, why not use a skim coat of gypsum plaster and paint over that.
I'm not blocking the air vent, the fireplace has been bricked up and my builder has drilled a hole through for an air vent but it needs a grille on it to cover the hole and finish off the wall. The query was that instead of putting on a plastic or chrome vent, I wanted to put a plaster vent on so that could be painted to match the rest of the wall....
OK. Well you should use a clay grill these paint up OK and will do the job. Do look at the weather proofing of the top - a lead cap or concrete plug to keep out the weather will make a big difference to the temperature of the chimney breast and will make the room/rooms adjacent far more comfortable. No point in loosing any of your heat up the chimney just to keep it dry.
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