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Discussion Starter #1 (Edited)
So we're getting close to the point of Tiling over our brick fireplace. Yesterday we applied some mortar to a few of the crevices (not the old grout joints) and the NEWLY-applied mortar was a bit sandy and flakey in spots.

Someone told us: "Mortar is not meant to be used as a skim coat. I suspect, too much sand in the areas that flaked. Brush off with a wire brush any loose mortar. Apply a cement based patching/embossing filling compound and smooth for your tile install. You might have to apply several coats to achieve desired smoothness. Speedfinish is one such compound:

SpeedFinish 10 lb. Patching and Finishing Compound-SF10 at The Home Depot "

So my question is more of what do you think of that opinion? We should NOT use basic mortar compound to fill in the grout lines? We'll be using a Thinset with a Latex Adhesive as the base for our Tiles.

so confused... Thanks in advance.
 

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Bagged mortar is designed for joints of 3/8" +/-. Typically tile grout is for 1/4" joints, and has a different aggregate design profile. Neither are designed as a skim coat. If the depth and width of the joints you are filling over is 3/8", use mortar with bonding agent, if less use polymer modified thinset. The linked product will probably work, but it is generally overkill (i.e. expensive) for a vertical surface.
 

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Discussion Starter #3
Thanks so much .. PS - what causes mortar to be "dry" and sandy and crumbles off when you rub a finger over is? Was it too WET or too DRY ???
 

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Discussion Starter #4
Follow up : Should we be "misting" down the surfcae of the bricks before we apply that SpeedFinish? What about when we start Tiling? Do we spritz down the area before applying Thinset?
 

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Probably too dry, i.e. the subsurface probably sucked all the moisture out too quickly, so you may have to mist.
 
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