Out here in New England we have older homes that in the earlier days used lead paint or had lead mixed in with the oil paints. One of the biggest problems was “Calcimine” ceilings. Definition: cal·ci·mine also kal·so·mine n. A white or tinted liquid containing zinc oxide, water, glue, and coloring matter, used as a wash for walls and ceilings.
At first glance, the ceilings look like ordinary latex or oil painted ceilings when in fact calcimine is acidic and if coated over with standard latex or oil based paint with fail almost immediately. One way to determine if your ceilings are calcimine is to check the underside of a paint chip. If it is chalky more than likely, it is calcimine.
Trying to scrape and “skim coat” (When you use joint compound to cover the cracks and defects caused by a failed coating) you will notice the joint compound starting to bubble. The acid in the calcimine is breaking down the joint compound. The best way to approach this is to scrape as much as the loose ceiling as possible. Then seal the original ceiling (as is) before repairing.
There are products out there that deal with calcimine directly. One such product is “Cal- Cote” I have used this product many times. A heavy coating will seal the ceiling and give you a surface to work with. Therefore, you can start skim coating and repairing all the defects after the Cal-Cote is dry. (Typically allow it to dry over night.)
After you have applied the skim coat to the ceiling you will need to allow the compound ample time to cure. Depending on conditions, (How humid is it?) I would allow the joint compound over night also. Then when sufficiently cured you can sand (wearing a mask and goggles) to achieve a smooth finish.
When all sanded, apply another coat of Cal-Cote and your ceiling should last quite a long time. One other thing, Cal-Cote is a very thick product. Unlike a latex or oil, based paint, which will typically cover 300 sq feet per gallon, expect to get about 200 sq per gallon. Following these steps will help in fixing those old calcimine ceilings. You can always ask your local paint store what type of product they carry and the coverage rate of that particular product.