So I did this to close up a tv cubbie. I had reclaimed wood on hand to build a mantle and corbels. The corbels are longer than I planned, but I think it will be fine once it’s tiled. I want the tile to go around fireplace and down below corbels. Would love it here your ideas for masonry.

White based plaster finishes don't compliment natural materials, such as your flagstone choice for the coping. Mind you, my opinion. Plaster finishes in the gray scale look best with natural stones and tiles that are of the same color scheme. Grout choices should be also in the gray scale. with gray grouts. NOT WHITE!!!! Glass tiles with blues, greens, grays (light and dark) look fantastic . Click on my Houzz page and see the photo is a gray Hydrazzo Hatteras Gray with Limestone coping and glass tile with aforementioned glass tile 1x1.

Salt is neither an acid or a base, so it does not require any neutralization. Apparently what has happened is that salty water got under the tile and moisture continues to carry the salt out. Any moisture under there will of course evaporate where it hits the air, i.e. at the grout joints, leaving salt behind. I am not sure this process can be sped up as it depends on dissolving the underlying salt and then getting water to carry it up and evaporate. Here's a stab at it: You could try removing it with a MINIMAL amount of water - scrub with a damp sponge. Then add a very small amount of clean water just to wet the joints. Wipe off excess water. Run a dehumidifier in the area to draw moisture out. When salt collects, repeat. EDIT: I just realized this may be a wall not a floor? Not uncommon to have efflorescence of salts on basement walls. Happens on some tile I mounted on concrete behind my woodstove. You will have trouble getting water to go IN to the joints if it's a vertical wall though. How good is the foundation drainage outside the basement wall in this area? Reducing the infiltrating moisture should help. And if you paint over a wall that's efflorescing, I doubt the paint is going to hold, regardless of whether it's salts from soil, concrete or water softener. It's the moisture that's the problem.

Which one is your FP that is what we need to see.