Coz you pour in some water, bout 60/40 water and paint. pour it in the extinguisher just over half full.
Do it up tight then repressurise it at the servo.
Ya gotta thin the paint, and you'll presumably want to reuse/keep the extinguisher.
The more air, the more pressure. (higher/further you can blast it.)
The more paint, obviously it will last longer...
But not enough air and it wont shoot, just dribble out everywhere, most likely all over you.... Like a big Ironwak can.
Ill give ya $50 for it.