What is the secret to creating a good mask for sandcarving?

The secret to a good mask: quality film print and a vacuum frame exposure unit. The vacuum frame exposure unit compresses the print film and photoresist, creating a quality photoresist stencil. 

Quality print film is a dark opaque print that no light can pass through during exposure. In addition to an opaque print, inkjet film is recommended as the print film. Inkjet film has no light diffusion and allows the ink to absorb onto the surface. Vacuum frame exposure will compress the print film and photoresist film, ensuring no light between the two products during exposure. This is best for detailed images. 

   —Rayzist