....for the skim coat? Would it be lime putty plaster, or gypsum plaster
(such as plaster of Paris)?

If lime putty, can I use the bag of slaked lime that's been in my garage
for a year or two (but not more than 2), or will it be too carbonated by
now? I *really* doubt that I can find quicklime.

My inclination is to use a setting-type joint compound, like Durabond 45.

Thanks,
Bob