tiling onto new plaster

Once plaster completely dry, PVA (5 to 1 or thereabout), and tile. I have done 5 bathrooms and 7 kitchens this way over the last 5 years and haven't had a problem yet.

IME standard tile adhesive sticks perfectly well to new plaster. It's designed that way

You have to check the instructions for the adhesive you use. None are likely to recommend PVA as it's water soluable, which is not a smart move in a shower. You shouldn't need anything on new plaster as the surface will be stable, and the adhesive will contain its own bonding agent anyway. Just check it's suitable for use on plaster (most are nowadays).

I would suggest using a waterproofer/plasticiser in the grout, such as BAL Admix GT1, although this is more important on untanked plasterboard walls.

Except when it's waterproof?

MBQ

Waterproof/Exterior PVA (which is really EVA) is only waterproof inside a mortar mix (and it's only a bit more waterproof than PVA). Other than that, I'm not sure what you're referring to.

If you want to create a waterproof barrier (e.g. on plasterboard), use one of the tanking products designed for the purpose. If you just want a bonding agent, that's already in the tile adhesive, and it sticks rock hard to anything (as anyone who's tried to get a misplaced bit off will testify;-).

OTOH I slapped tiles onto fresh plaster within days with no further preparation at all, and they are fine also.

A lot probably depends on the tile cement. I found 'evostik waterproof' which is like 'no more nails' in a tub. Its the best yet.

That's why the tiles and the grout are there. To keep the plaster walls dry.

If ANY water gets to plaster you are in deep doggy-doos.Never mind the PVA.

You shouldn't need anything

Thanks, nor was I really, hence it was a question rather than a statement.

MBQ