Protect the Surrounding AreaTape off area around the fireplace and place a drop cloth on the floor to catch any drips.
Clean the brickClean the brick if it has soot or dirt on it. Use a vacuum to remove any ash or dust. And if there's any blackened areas, you'll want to remove as much as you can with a scrub brush and a heavy duty cleaner. Work in a circular motion if the soot is heavy. Rinse well and allow to dry for at least 48 hours. (Our fireplace hasn't been used very much so it had minimal soot on it.)
Mix the paint with waterFor this rustic French technique, I added a small amount of the paint, probably a cup and a half, to my handheld paint cup and thinned it down with water at a ratio of roughly 1 part water to 4 parts paint.NOTE: If you're not sure how opaque you want the paint, start out with more water than what I used. You can always make it more opaque by adding more paint if you decide that's what you want.
Paint the mortarUse an angled brush to paint the mortar. I used a professional brush and dragged it across holding the brush longways as shown. Be sure to offload any excess paint. You don't want too much paint on your brush or it will run down onto the bricks below it.
Lightly brush the paint over the brick surfaceAfter I painted the mortar, I dipped the brush back into the paint/water mixture, offloaded most of it back into my handheld paint cup, and lightly dragged the paint brush across the brick at an angle. Our bricks have imprints of twigs and leaves and I really wanted to accentuate them. I was basically skimming the paint over the surface, deliberately keeping it from seeping into the imprints so they would be darker and contrast somewhat against the light paint.