Priming wood moulding should be done whether the moulding is used or new. Primer covers up raw wood or existing stain and provides a base surface over which to apply paint. The first step to priming moulding is to make sure it is thoroughly clean and free of burrs, snags, or splinters.
Used or New Moulding?
Some crown moulding is so attractive, it's worth using over again. After you've cleaned, scraped, and/or sanded the used moulding, use a stain-blocking primer to cover the existing color and prevent it from leaching through the painted finish. New moulding is typically sold bare, so all exposed parts of it should be primed and allowed to dry before applying paint.