Thanks MRunknown! I've added your Sandblaster to our Media Blasting category, as well as to your builder page: MRunknown's Homemade Tools. Your receipt:
Wow! great! thanks very much for that! I made it yesterday slightly differently with a pressure gauge at the "T" intersection. It seems to work quite well, i will test it again today, I learnt something important about putting threads in fire extinguisher tanks - you cant, they are only about 1.5 mm thick and the threads will quickly get ruined. so to solve this, i had to make a very narrow 1/4 inch nut that could fit down the bottle neck and down to the bottom with a socket wrench. now it will never get shredded again. also, i didn't use a tube for the exit, i just used the hose barb on its own. yesterday i only tested it up to about 90 psi and it seemed to work well (but i have never used a shop-bought sand blaster so i couldn't compare it to anything) now that its all air tight i will test it to 150 psi.

What if you got rid of the "homemade hollow bolt connection" and Tee'd the hollow tube off the air inlet? That would keep you from having to put another hole in the tank. Might make it hard to repeatable fill with sand though.

If you have not seen it already, checkout the video called "Sandcarving pressure pot-101" on youtube. It has a lot of information. What was particularly interesting is how sometimes you might want different pressures in the media tank vs the feed line.
Thanks for the ideas, i thought of that, but i didn't have a "T" hose barb intersection to use, and i wanted to buy as little things as possible (because that means convincing parents and compressor fittings are pretty expensive around here). the only part i bought was the inside threaded hose barb for the exit.

I'll have a look at that Video, I like the idea of different pressures, but that's pretty complicated if you think about it, that means either 2 compressors or 2 pressure regulators, and i wanted this to be a nice and simple thing that works reasonably well.

NOTE: This was designed by a 14 year old (me)! if you build it, use it at your own risk!

Thanks for your ideas! i don't have the resources at the moment to do them, but someone else might!

Thanks.
MRunknown