Here are the tricks I know about repairing plaster over lath. Small holes as JoeD said use joint compound. Tight cracks use mesh tape then joint compound feather it out and sand. Loose cracks where you can feel plaster move away from lath, use thin plaster washers and drywall screws along the crack screw into lath and pull the plaster back up tight, tape, compound and sand. Areas missing as mentioned above half inch drywall is about right thickness. If to thick use 3/8 trim plaster careful back and screw a patch in place, tape seams and use compound or real plaster mixed with water. The compound shrinks when it drys and if put on heavy takes a long time to dry and takes many coats. Repairing plaster is much quicker for large gaps. It is very hard and tough to sand down. Try and get it as flat as you can. I have then used compound to get it perfect. Around switches and outlets quite often the outlet wobbles around and lath is split or loose. I make up from half inch plywood rectangles about 3" bigger on a side than a outlet box with a hole in center for the box. Carefully cut back the plaster and screw in the plates with a screw into each lath end around the outlet. Them tape and compound. Ceilings are mostly nothing to go around and no trim, maybe one ceiling light. If they are bad or loose rent a drywall lift and go over the whole ceiling. Before you do that figure out where the joists are and snap lines. Poke holes if needed and probe with a coat hanger wire to make sure. Old houses were not 16" on center. You want to use screws long enough to go thru plaster and lath and into joists. Finish ceilings with tape and compound. A trick I use is to mix joint compound with latex paint and thicken the paint right up. Not so much you can't roll it but thick enough to really lay on a thick smooth coat. I do it on walls and ceilings. Ceilings that's the final finish. Walls I paint the color and sheen I want over it.
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