Speaking as a former USCG architectural technician, I would recommend that when you've finished the CMU work and have allowed the mortar to cure, be sure to seal the outside of the CMUs with a concrete sealer. If you don't, over time you will see the salt from the concrete (that the blocks are made of) and the mortar driven to the inside and crystalizing on the surface. This is called "efflorescence." This is the same preventative step that should be taken when a basement is constructed. Putting a sealer on the inside surface after the fact is useless. The efflorescence will build up under the sealer and force it off. This was a constant problem in the light house towers that had failing outer protective coatings (usually thick paint), and much newer brick buildings (boat haul-out structures and such) that I dealt with.
