I tried to 'skim coat' a floor-to-ceiling mid-century fireplace - red brick over concrete block construction. Here's what I tried without success (looked great for a few days then fell off): thinset (acrylic fortified) over the brick, then a finish coat of portland cement steel trowel finish. The finish coat went on after the thinset had fully dried - a few days or so. I wetted the thinset coat well before applying the cement finish coat, however. I fully expected this to work. The 'polido' finish is common: fine cement stucco over concrete block with a final portland cement slurry troweled over to a polished finish. Drying conditions affect adhesion of the polido so I was careful to manage the drying of my fireplace polido accordingly. As it turned out, some sections of the finish adhered perfectly but much of the work is cracking and falling off.
I'd appreciate guidance on how/what I can get to work with the dry thinset coat and why my polido finish is falling off. I'm assuming the acrylic prevented a good bonding with the cement. Not sure why parts of the finish are good and others not. If the acrylic is indeed the problem what should I use as a new intermediate step to bridge to the polido coat? Thinking Sika something.