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So I have a wall that I had to replaster. Usually, what I do with plaster walls is, because I am never happy with the smoothness of plaster alone, apply blue lightweight drywall joint compound and sand it before painting, that way a plaster wall looks as smooth as drywall.

Now, I know the theory that lime-based plasters (which is exactly what I use) requires up to a month or longer to cure and should not be painted before. What I am not sure is how things change if joint compound is applied: does the plaster need to cure on open air regardless of paint or is it just bad to put paint directly on plaster before it is cured? Because the latter case would not apply if there is a layer of drywall mud between the plaster and paint.

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The curing process for lime plaster (Calcium Hydroxide) involves the plaster absorbing and reacting with Carbon Dioxide in the air to form Calcium Carbonate.

Therefore no air impermeable material should be applied to it for the length of the cure, about 3 months. Applying joint compound over it will prevent the curing process, with or without paint.

See: http://www.buildnaturally.com/EDucate/Articles/Lime.htm

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but what happens if it doesn't cure? –  amphibient Jul 28 '13 at 2:26
The plaster will never completely "harden" without this reaction and will flake and powder. It is possible anything placed over it will fail to adhere, and the material will be softer and more prone to denting and cracking. –  mfarver Jul 30 '13 at 15:12
But the plaster hardens after 24-48 hrs and past that, i can't notice any changes taking place in terms of state. Can I assume once it is hard it no longer cures, therefore it is safe to cover? –  amphibient Jul 30 '13 at 18:23
The plaster dries out becoming solid, but it will not achieve its full strength unless it is uncovered for 30 days to 3 months depending on thickness. Concrete becomes hard enough to walk on in a few hours, but you can't put significant load on it for 4 days, and it won't achieve full strength for 28 days. –  mfarver Jul 30 '13 at 22:02
Alternatively, you could repair a plaster wall with actual Gypsum plaster (also know as plaster of Paris), instead of lime plaster. The reaction that cures gypsum plaster does not require air, it is water based like concrete. "Setting" joint compounds which contain it are stronger than normal joint compound and usually labeled with a work time (5 min, 15, 90 200 etc). they should be available at most home stores –  mfarver Jul 30 '13 at 22:21
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