Repairing Old Ceilings
To save an old plaster ceiling that has only a few cracks or holes, a little time and a few materials will do the trick.
Patch up minor cracking and small holes with some quick-drying compound. Apply it with a taping knife, smooth it over, and then, when it’s dry, sand it lightly with a sanding block or an orbital sander using 80 to 100 grit sandpaper. Finally, add a thinned, finished coat of drywall compound.
The first step in repairing a bigger crack is to undercut it with the sharp end of a beverage opener. This provides a better bond for the quick-drying compound.
Apply a thin layer and then place a piece of fiberglass tape over the crack. This will prevent the crack from spreading or reopening. Wipe another coat of compound across the top of the tape, feathering it in as flush as possible with the wall.
Let it dry for a day and then lightly sand it with a sanding block or an orbital sander. After this, add another layer of compound and feather it out flush with the wall.