Wet sand blaster

No, but I did have a proprietary sand blaster attachment for my (old) pressure washer. Pretty much useless

No, but I have used the one for my presure washer. It works OK to shift some sorts of green growing gunk, but it has no power for any sort of mechanical work, de-rusting etc. It's also a pain to clear up afterwards.

For cleaning metal, the only thing I've found useful is a pressure-pot gritblaster. You can make these yourself (steel drum, pipe fittings)

No, but some clues. They work on the venturi principle. High pressure nozzle firing across an angled intake pipe. Output nozzle needs to be hardened steel as a minimum, ceramic is best.

You need a big FO machine to effectively run a wet sand blaster. 200bar @

I'd say about 30 l / min actually (but not quite that pressure) A pressure pot can be used usefully with a 10l/min compressor, but you're not getting the same sand delivery rate. If you have small enough nozzles, you can even throttle back to a rate that allows continuous running, still on a D-I-Y compressor. Bigger nozzles though need enough airflow to keep the sand moving, so you're limited to an intermittent duty cycle.

Sorry, when I said 'machine' I meant pressure washer :-)