I had the same problem with a family gravestone on which the gold leaf on the engravehad lettering had deteriated with age. I asked quotes from a couple of local monumental masons to refurbish. I was shocked and annoyed at the high prices given and decided to tackle the job using gold coloured enamel paint, despite being told it would not work. The preparation was most important, i.e. a thorough cleaning of the engraved lettering. There was an amount of excess paint on the edges but as the marble was smooth this excess was easily removed using a Stanley knife blade. That was three years ago and it is now beginning to show signs of deteriation so it's time to do it again. It took me half an hour and saved me megga bucks. Use good quality paint! I also intend to coat the finished painted letters with laquer for added protection this time.