I bought one of those siphon tube sandblasters at Central Tractor several years ago. The first sand I used was from a sandblasting firm in a town near here. I only bought a couple bags of sand, but the guy cautioned me quite a bit about breathing in the dust. They had several grades of sand, and sold me the finer stuff when I told them what blaster I was using.
I also got lucky about two years ago when there was quite a bit of sand left over from sandblasting operations at our fab shop. The guy in charge just wanted it to go away. OK.
The setup I have is best suited to small projects. It works well, and with the larger granules of sand it is pretty aggressive. It is quite slow though. I did a 5' RFM a couple years ago, and the blaster was too slow to put up with. I ended up using a real stiff cup brush on an angle grinder and sanding discs for most of it, then blasting where these two didn't reach into.
One thing that helped the blaster work better was using a plastic jug for the sand that can be carried in one hand while holding the gun in the other, and trimming the hose to the pickup tube as short as practical. This made my puny gun feed better. Also, if this is like the one you got, drop the tube in the empty bucket, then add sand. It will help you avoid plugging the air passage in the pickup tube....................chim