Buying primer versus paint and primer all-in-one, what’s the difference? Priming is a general term. It’s not specific to one product; all it means is that depending on the substrate, it will prime the surface in preparation for a specific top coat.
Paint and primer all in one mean that the grade of paint may be high enough to get it done in one coat when covering a previously painted surface. It won’t work on raw surfaces such as Metallic surfaces (steel, sheet metal, etc.), raw wood (tannin Bleedthrough), water damage, smoke damage, or fiberglass. They all require surface-specific primers. It essentially only works over standard latex paints.