its best to contact a local paint retailer about this as he will best understand the application and local suppliers of the necessary materials.
It is know to be concrete, gunite or shotcrete.
No, plaster of paris is not strong enough for concrete.
First you have to determine if is in fact a painted pool. If it is a plaster pool then usually there is no paint unless it was painted because of age and damaged plaster. You most likely do not want to paint a plaster pool - have it acid washed to clean plaster. Not sure how to determine what type paint. You would have to have the paint chips anaylized to determine if it is a chlorinated base paint or epoxy paint. Ken
You drain the water then plaster the walls from inside
Frescos are painted directly onto wet plaster.
Gunite is covered with plaster. It is sprayed on concrete sorta. They mix it in the hoses and it is blown on. You can go to youtube and see them gunite a pool. I am only aware of plaster, vinyl or painted pool surfaces. The vinyl is just like thin rubber.
No, salt does not damage the plaster. Improper water balance will.
Frescos are painted directly onto damp plaster on walls.
Plaster itself is not poisonous. Some people are allergic to it but generally you will only have a reaction to it if you consume it. Plaster of Paris may cause breathing difficulties if inhaled. Wall plaster is harmless once it is painted and/or covered, and contact with it before that is fine. Painted wall plaster is safer as it does not release dust.
You don't need to tile a pool, if you use a good waterproof concrete render and paint it using proper swimming pool paint. (don't use emulsion)
Acid washing can be used on concrete and plaster. Swimming pools are commonly acid washed prior to resurfacing. It is also used on aluminum to remove and neutralize oxidization.
No Plaster of Paris is too soft and too porous to be useful in any thing to do with a swimming pool.