Hello
for old work patching is part of the job...
the best repair for old plaster is durabond 90 or 45 (hot mud) mix as thick as you can, and let it cure untill almost hard but can be shaved with a putty knife for a big hole use metal lath
a sheet of metal lath and a bag of durabond will cost under 15 bucks you don't need to carry much, just a few small pieces of lath and a coffee can of durabond will be plenty for a service truck.
incidently for repairs in wood molding I prefer rock hard water putty
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