I am grinding out mortar joints to a depth of 3/4 inch for repointing. How clean should the surfaces of the brick be? Is it all right to leave some mortar on these surfaces?
No. All old mortar should be removed from the brick surfaces to the depth of 3/4 inch. Mortar left in place will prevent the new mortar from bonding to the brick, which may inhibit proper performance of the new pointing. Mortar in the portions of the head joints that come in contact with the bed joints can't be removed with a grinding wheel without damaging the brick; it must be removed by careful chipping with a pneumatic chisel or hand chisel. Take care to avoid spalling of the brick.