In the past we would lay the marble out and seal the front twice before setting the tile then twice after it gets grouted . One day the guys forgot to seal it and just glued it up . We had a few glue spots on the tile from finger prints but it cleaned off OK . Then the same guys forgot to seal the marble and grouted it up . It still looked about the same after it was grouted so for the last 15 years we seal twice at the end .