before we make a mess
 

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.

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.

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 ???

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?

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