The addition of sand to the lime putty adds strength and prevents cracking.It also helps provide a tooth to adhere to for the second smoother coat of plaster.The texture of the sand does not have to be visible in the final image,as each later of plaster uses progressively finer sand.I'm experimenting with allowing the texture to show in the final image as well as allowing the marks of the tools used to show,as in the impasto oil painting technique.Traditional fresco calls for a smooth surface,experiments with digital media and traditional fresco,as well as combinations of fresco and secco techniques caused me to try some experiments with texture as well as how and when pigment is added to the plaster,for various effects.