I want to fix a damaged wall, and make it flush with a wall above it, which is about an inch more narrow. To do this, I was thinking of putting a layer of a cement-sand mixture on top of the existing wall.

Parts of the wall have some form of white plastering on them. I can't tell what kind (I'm a complete amateur) but it seems to be pretty solid (nothing comes off when scratching it) and water-resistant.

I realize this can't be answered accurately without more detailed info on the plastering which I can't provide, but could you hazard a guess from this (bad) photo how the areas with the white plastering should be pre-treated?

Would I need a deep-penetrating primer (I have some of that), or do I need to remove the plastering in any case? If so - how would you remove it?

Is there a mechanical trick for making a layer of cement stick on a wall with an unknown surface?

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    An inch is an awful thick amount, if I wanted to keep it all concrete I would put some thin furring strips up and attach cement board to that then a light skin coat. – Ed Beal Feb 9 '17 at 23:10

Existing wall > 3-6 mil plastic > vertical furring strips > cement/green board (drywall) > spackle/stucco/whatever

Because old concrete doesn't adhere to new concrete; especially vertically, with sufficient confidence in many cases.

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