Should custom masonry mixes first be mixed dry?
Dry mixing all the ingredients before adding water ensures a complete homogenous batch. Mixing dry allows you to see how complete the materials have combined. Proper blending of the mix is necessary in order to produce a strong and workable mortar.    Of course if you are using a powered mixer you would just shovel in the proper amounts as it is turning and add the correct amount of water. No need for dry mixing.