I'd done hundreds of patches with few problems, using just the plaster and water. Though thicker patches did tend to crack, if they were not sprayed or dampened every couple of hours, so they cure slowly. This seems less of an issue recently, the materials seem to have been improved some.I would not prime the old plaster first. It will inhibit the bond between the patch and the original plaster. You could dampen the surface of the old plaster to keep it fom drying out your patch too quickly if that's a concern.