Mortar skim coat coming off as dry powder?

Hi guys

I am re-tiling my shower which was previously wet-laid. The surface under the wet lay is heavily scratched and not as even as I would like. On advice of a builder I used a premix mortar to fill around an exposed shower rose and to skim coat the walls. The thinnest areas were perhaps 3 mm thick up to 5 mm and the surface was floated even when green.

Today I checked and found that most of the mortar is rubbing off as a dry powder right back to the original surface. I have only checked the skim coat not the thicker areas around the rose. I have used mortar before bricking up holes and around pipes in thick quantities but never as a skim coat. I wetted the wall prior and the mix was runnier than usual to allow better flow.

What would cause this? Is it the wrong product for the job, is it a premix issues etc

I have since been advised to use tile cement as a skim coat.

Cheers


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