My homemade mortar sprayer has been working well for my experiments with burlap-crete.( http://papercreteparadise.blogspot.com/2012/08/a-low-rent-mortar-sprayer.html) However, when I tried to replace the air gun with one (I thought was) just like it, I found that the threads of the coupler inside the PVC fitting were too short to engage those on the air gun. Though both were purchased at Harbor Freight, the air guns were slightly different. Which means not every air gun will work in this system without some additional retrofitting. I had to grind away some of the thickness of the PVC with a Dremel tool, and also cut the little curled ring off the new air gun to make it work. Also, I’d used a Clorox bottle at first because I happened to have one. A gallon Concrete Bonding Agent jug is sturdier and is a tighter fit.