Apparently, the US Coast Guard believes the surface of a sailor matters. They are ready to implement new regulations on permanent body art. Other military branches also have some similar rules.
“In the seventeenth place, the Word of God is not rightly divided when men are taught that the Sacraments produce salutary effects ex opere operato, that is, by the mere outward performance of a sacramental act” (C.F.W. Walther, The Proper Distinction Between Law and Gospel reproduced from the German edition of 1897 by W.H.T. Dau (St. Louis: Concordia, 1986), 4).