You ought to satisfy yourself that there is no damp problem eg. rising damp downstairs room, or damp penetrating from outside esp. round windows. You don't really want to go ahead with plastering if there is a problem. If there isn't, then there's little point getting rid of the smell now - the plastering will create its own damp, and its own smells, and you'll need to give the room as much ventilation as poss while the plaster dries.
If you are happy there is no damp lefty, fresh air is the best. The musty smell indicates damp air, either having been shut up for a long time, in ingress of damp somewhere. You could even try a dehumidifier for a day or three.