When I started working with papercrete I thought spraying would be a good method to use. I looked around at various commercial sprayers but found that none of them fell within the budget I had, especially for something I wasn't sure was going to work.
Looking around the www, I found a couple of home made sprayers that were used for various things, so I studied the basic design requirements and set out to make a sprayer of my own. After making some sketches of what I thought would work, I made a trip to the local plumbing supply house and traded $30US for a bag of parts. I found that in many cases the supply house did not have exactly what I needed so I had to combine a couple of parts to make the one I wanted.
Here's the completed sprayer. I did not glue any of the parts together, making it easy to take apart for cleaning. I found that the parts stay together well just from friction. The discoloration is caused by UV exposure and various spills. Your sprayer should be shiny white, at least when you first make it.
This is the hopper, made from three feet of four inch PVC pipe, a 4 x 2 reducer, and a 2 x 1 1/2 reducer so that it would slip fit onto the 1 1/2" PVC pipe on top of the sprayer. The hopper could be made from a variety of things as long as it fits on the top of the sprayer. I even saw one on the www made from an old metal teakettle.
In this picture the 1 1/2" tube on top of the sprayer, where the hopper is attached, is visible.
Here's the feed end of the sprayer, starting on the right with a quick disconnect air fitting, connected to a 3/4" brass valve and an 8" long 3/4" brass nipple. All the threaded fittings got teflon tape.
The brass nipple is threaded into a 1 1/2" x 3/4" threaded reducer that is inside a 1 1/2" PVC coupling. On the other side of that threaded reducer is a short 3/4" brass nipple, a 3/4 x 3/8 brass reducer, and a 3/8 x 1/8 brass hose barb.
The rest of the sprayer is a 1 1/2 PVC "T" fitting with a 1 1/2 x 3/4 reducer as the nozzle and a short piece of 1 1/2 PVC pipe for the hopper attachment and for the coupling containing the brass fittings. Ideally the end of the hose barb should fall under the front (nozzle side) of the hopper attachment.
Here is a top view looking into the hopper attachment, showing the relative position of the end of the hose barb. Mine fell a little short of the front edge, but the sprayer works fine.
None of the fittings are critical, substitute what you have or can find. The critical thing is to get enough air, and the relative position of the air outlet (hose barb) with the feed from the hopper, and the nozzle. Even this is a bit forgiving.
My 7 1/2 HP compressor is really too small to do the job, only supplying 7 CFM at 40 PSI. I can spray for about fifteen minutes and then must stop to let the compressor catch up. A larger compressor would be good. Using higher pressure makes the spray go farther but also depletes the air supply faster, causing more frequent breaks.