I built my own blaster about 15 years ago from an old water heater tank. It's ugly as sin, but as effective as any of the commercial unit's I've seen. The design is the same as Janiluo has shown, and it holds up to 200 Lbs of sand.

I think the tank was rated at about 300 psi new, and after I welded on wheels and legs and a 2" fill bung in the top, I pressure tested it to 200 psi.

It sounds like your running way too much sand through your unit, doing some rough calculations your using 5-7 times the amount I use. The sand is supposed to come out in a fine mist.

My working pressure on heavy parts (manifolds, brackets, hinges etc) is 100 psi. I've done a lot of sheetmetal with no problems, but I only run 75-80 psi and move rapidly so as to minimize heat buildup. I've even done hard plastic pieces such as arm rest bases, window garnish etc (65-70 psi about 18" away) to clean and rough the plastic up to take paint (this is not for the faint of heart or the beginer). I run an 80 gal 2 stage compressor and I run out of air pressure after about 20 Min ( I usually get well over an hour of actual blasting from 200 lbs of sand, depending on the air pressure and how much sand I'm actually running through).