Where the plaster is falling off the walls and breaking up on the bottom it will have to be removed. Then test the adhesion of the remainder by tapping it with the handle of a knife or large coin. If it sounds hollow (it is quite obvious) then it will have to be removed also, and if it sounds solid it can remain (draw round the solid parts with an indelible marker pen). It is quite acceptable to patch the plaster, you do not have to remove the good solid bits. However, if the solid bits have cracks, fill them with butyl rubber or concete repair material as previously explained, do not wipe new plaster over them. After patching or before complete new plastering, run a bead of butyl rubber horizontally along the joint between the walls and the floor, and vertically at the junctions of the walls. I meant to mention this before, it is a weak spot and there is often a hidden leak there. The butyl will assist the epoxy to bridge. And then, yes, you must paint the whole of the pool with primer and epoxy paint. When you come to sell there should be no significant loss of colour, but you should hose down the epoxy and give it a wipe and polish with ordinary window cleaner, it will sparkle like new and add $5000 to the value of your house!