Rough, uneven stones are our biggest challenge. We blast natural sandstones for a funeral home on a regular basis. These stones have to first be washed thoroughly since the irregular surfaces hold a lot of dirt and dust, which in turn prevent stencils from sticking to them. After washing, the stone has to dry for quite a while to avoid “sweating” after the stencil application, which can result in lift-off during blasting.
Since all photoresist has a stiff cover sheet on one side, it is not the best choice for this substrate. To blast these stones, we use a soft vinyl material that comes in a roll that we pre-cut with our plotters. Before plotters, we hand-cut the stencils from this material. The 11 mil-thick resist is soft and flexible, which allows it to adhere to uneven surfaces without a problem. The adhesive is also strong enough to stick to these porous surfaces for prolonged blasting to achieve depth. For these stones, I usually blast at 40-50 pounds of pressure while holding my nozzle about 10-12″ away.