The white of acrylic paint and the white of the gypsum are different. The feel too; the plaster is smoother than the paint. Some tiny, tiny spots might be missed (unlike with black), but they won’t show.
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I like to base coat no matter what. If nothing else it makes to absorption rate of the paint job more even than it is on bare gypsum. And dragonmedley is right. The gypsum is a different color than the paint. It looks off white when compared to white paint.