thats quite a handful of questions.
in true authentic plaster (lime based - not to be confused with "plaster of paris") no chemicals are used. just lime (calcium hydroxade) and aggregate (river sand or marble meal/dust/sand or the mixture of the 2)
if applied properly sealer is not needed actually it would shorten the life of the plaster, as plaster cures it "seals itself" via chemical reaction of CaO2 from the air combining with CaOH2 (lime - calcium hydroxide) resulting in CaCO3 (calcite/lime stone/marble).
So technically the wall becomes stone once again:
some more details here:http://frescoschool.org/fresco_introduction_what_is_fresco_painting.html
The plaster "breathe" the moisture from the air and continues to cure itself for hundreds of years, it is estimated that very thick plaster walls of 500+ years old cathedrals in italy are still curing...
It is possible and sometimes needed (for a pool or bath) to use soap (to make it waterproof) Wax (to polish and additional water proffing) or both. However soap if applied while plaster is wet/uncured creates chemical bond/seal and becomes one with the plaster, wax needs to be applied when the wall is completely cured and dry, it will stay on the sirface and will needed to be reapplied regularly.
Chemical manmade solvent based osealers will destroy plaster in a few years. More contemoporary - water based sealers will last longer but eventually will fail because they stay as a surface layer (even with a slight penetration) and do not create a true mineral/chemical bond as natural materials applied while the plaster is still wet/uncured (lime is not yet turned into CaCO3 (stone) uncured).
learn fresco at http://FrescoSchool.org
fresco painting video tutorials: http://www.FrescoSchool.com
Fresco School Video Channel
Contemporary Fresco Gazette
commission fresco or mural http://iLAdesigns.com
Contemporary Fresco Painting Resouce Center