Fixing mortar joints on cultured stone ???
Typical joint in brick or stone is much deeper than it is wide. In fake stone, the joints is as deep as it is wide, or often, wider than deep. This leads to the mortar drying out very fast, not leaving enough water for the rehydration reaction that the mortar needs to properly set. In addition, the concrete that the fake stone is made of also absorbs water like a sponge.
Typical symptoms of this condition include the mortar hardening up within a couple of hours, instead of a day or two. The surface may be dry, white and powdery so that the color of the mortar never actually develops. Depending on the width of the joint, which varies in a stone application, the color will vary depending on the rapidity of the drying out.
On exterior applications this a serious cause for concern as the mortar will crack over time, it will not be properly adhered to the adjoining fake stone and generally become a poor water barrier. The only real fix is to remove all the mortar and start over. Where I have caught contractors doing a poor application, they have been able to scrape out the offending areas of mortar fairly easily, since they were doing it only several hours after they started.
I have never done the application that you describe as for my projects that would not be acceptable, the improper application of the mortar would cause the work to be rejected. There are concrete color stains available out there, Sherwin Williams comes to mind.
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