The way I made a coarse sprayer to do blood spatter/spray in a couple videos was by splitting the compressed air. I had a plastic hudson sprayer (with a side port that you could put an air line to), and took the trigger grip off so it was just an open tube leading out. Incoming air would go into the side fitting, and the pressure forces liquid up the tube and out. Left alone, that will give you a squirt of liquid (instead of a fine spray, since I took off the sprayer end). For splattery spray, you want the liquid to be forced through a side-connection into a slightly wider tube (the "carbeurator") that has the other branch of the compressed air blowing through it. Put separate control valves on each branch of compressed air (the one blowing directly through and the one going to the liquid supply; you'll need to adjust the two branches until you get the spray you like). The wider the carbeurator tube, the coarser and splashier the stuff coming out will be. If it's too wide, the air won't have enough pressure to really blow it out any distance.
If a little finer spray or a jet is fine by you, just leave the sprayer handle on the hudson sprayer, run your air pressure to the tank, set the sprayer valve on coarse spray/stream, tape the trigger down to "on" and forget about the carbeurator thing. Though I prefer my way because I'm paranoid about the spray tip flying off under pressure. Not that it would, but I'm skittish that way.
