H F Sandblast Cabinet won't suck

I bought that small gun with the qt size hopper on the bottom. Blew through that in about 30 seconds so I just put a plastic hose on the pickup tube and shoved it in a bag of playsand, son of a gun, it works! I hear sand is terrible for your lungs but I shoot it into a closed box with a glass sight and a shop vac sucking out the other side. I like blasting, even if its only for about 30 seconds at a time as that is all my compressor is worth, sort of a 50% duty cycle....That black grit (garnet?) works good too. Mine gets gunked up as the air supply is not dry. I have to take it apart and literally chip out the media from the suction tube where it crosses the air path.