Once wood is sandcarved, a paint-fill application is recommended to provide contrast. If you do not want to add color to the etched area, then you can apply a clear coat that will darken the image, creating contrast.
To do this, leave the photomask on the surface and clean the etched area. The sandcarving photomask becomes the painting mask. Apply the paint in light coats from different angles to cover the etched area. There are many paints available to use on this surface. Spray or airbrush applications work well with a grain texture.