I have seen many architectural specifications stating that bed joints should not be furrowed. As a mason I was taught to furrow bed joints to distribute the mortar better. Why do so many architects prohibit it?
Furrowing bed joints is discouraged because, when it is excessive, it can reduce contact and create voids. Furrowing is the process of dragging a trowel across the center of a bed joint to form a depression. In general, lightly furrowing joints has little effect on the filling of the bed joint. Deeply furrowed bed joints, however, will not be filled when the unit is laid. This creates a channel that can allow water to move horizontally or collect within the wall system. With practice, units can be laid effectively without furrowing the joints. If you do furrow joints, be sure to limit the depth of the furrows to avoid any potential problems.