I have no experience, but could mention that at one of Europes (at that time) largest steel foundries, a sandblasting holding booth for cleanblasting casted steel objects gave problems with very fast detoriation and excessive wear of the booths steel walls. Solution: walls clad /protected with rubber.
(Soft sealing NBR or EPDM clad valves if suitable? Pinching valve?)
Look up what they do with slurry - The issues are similar, but slurry is much more common. Either soft material (rubber, EPDM) or very hard (ceramics) seem to be the solution.
Not introducing sand until you absolutely have to would also seem like a good idea...
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
Umm, you turn the air on and off with a valve- a cheap commodity valve- regulate the flow of the air using a pressure regulator, and EDUCT the sand into the flowing air. You do NOT try to regulate or stop the flow of the air + sand mixture in any sandblaster I've ever seen.
If you are referring to a metering valve, thare are a few good ones on the market. Assuming that you are referring to a pressurized blast pot, there are two primary systems in use. 1) the pot is equipped with an exhaust valve and an inlet valve. Commercial units have a mushroom valve that closes the fill port. The exhaust valve is closed, the inlet valve opened, the inlet air closes the mushroom valve and blasting starte. A pinch valve on the blast hose can be used to start and stop the blast without depresurizing the pot. Valves can either be manual or pneumatically controlled by a deadman valve at the nozzle. A very good site for more information is
