We spent 6 years patching plaster and fixing cracks as re renovated a 100 year old home. We learned a lot, and the big secret that helped us tremendously was BONDO. you know that fiberglass enhanced sticky stuff used for fixing cars- My husband has worked on car bodies, so he was familiar with the stuff. Using bondo was great for several reasons. It is strong, can be applied into large cracks and holes without the need for layering like spackle, and it dries relatively quickly. Read and follow the directions. Remove all loose plaster from the holes. Apply the bondo, and once it is mostly dry, smooth the surface (using a tool I called a cheese grater- it is maybe 2" by 7" - it is flat and can be held or fitted into a hand tool. smooth the bondo enough so you are happy with the surface texture because once it sets you will need a sander to change the surface ares. THEN apply a thin coat of spackle once the bondo is dry. The spackle is so paint will stick to the surface. Good luck.