The Secret to Sandcarving Irregular-Shaped Objects
Achieve your desired depth on irregular porous surfaces, such as rocks.
In most cases, remove the plastic liner from the film for the most flexibility of the photoresist stencil. For porous surfaces such as rocks, heat the surface of the rock until it is hot to the touch. This will cause the photoresist stencil to adhere to the irregular porous surface.
-Billy Willis, Rayzist