Tile is always done before the plaster application.

The tile border's function is to make it easier to clean the surface of dirt and debris.

When the tile is installed, the bottom edge allows the plaster to be feathered in for a seamless transition.

I'm not sure what you mean about the tile glue sealing the surface ??????

The whole pool will be tiled, not just the border. The plaster is for under the tile. one contractor says plaster is unnecessary & that the glue under the tile will be enough to waterproof the pool

I'm still not completely clear as to exactly what surface you presently have.

My understanding is the pool was sandblasted to remove the paint. Is the original concrete shell's surface exposed? Or was the paint over a plaster finish?

The proper method to tile a pool's surface whether it's the entire pool or just the waterline border, is that original concrete surface (substrate) must be exposed.

Any paint or plaster must be removed. When I say plaster, I mean a cementitious material that is troweled onto the surface. Popular plaster manufacturers these days are Diamond Brite, Pebble Tec, Hydrazzo, Wet Edge, etc.... Years ago it was called Marcite or a marble finish.

If there is plaster under the paint, it must be removed. The best method is a water jet blaster. It will remove all the plaster and expose the substrate. Trying to chip the plaster off is pointless.

This is a specialized piece of equipment, and this process can be expensive. But, it will remove all unwanted material.

Generally speaking, proper methods include that the surface must be prepped with a water-proofing material and layers of special mortars to provide a sound base to apply the tile.

The surface must be sound and free of loose material. Any cracks or fissures must be addressed.

Laticrete is one company that has guidelines for installing tile specifically for pools. Mapei is another company.

Some contractors though have found other ways to provide a lasting job.

Tiling and entire pool will be an expensive process. The job is labor intensive, and it's not done in a day or so.

Most tiled pools use glass tile which can be expensive in itself.

The contractor saying the "glue" makes me guess an adhesive is the plan. That's not advisable.

Adhesives/glues are typically used just for quick repairs of tile just to last the season before completely re-doing.

I hope this information helps.

Thank you, it does. The pool has been sandblasted down to the concrete shell. I live in Europe so much of the brand info is not geographycally relevant bit I appreciate your advice