Make the remaining parts as per the drawing. Note that two sizes are shown for the hole in the paint nozzle. Originally, I made the unit with the smaller hole. I made a second nozzle with the larger hole to spray some particularly heavy paint that was not supposed to be thinned. I've been using this one since. I can't make up my mind whether this gives a poorer surface or not. It sure can move paint!
No trigger valve is shown on the drawing .. Buy a small air blow gun (I got mine surplus for $0.50) and a 1/8" pipe to tubing adapter. The tubing end of the adapter should fit the nozzle end of the blow gun - you don't need the nozzle. The 1/8" NPT end screws into the back end of the sprayer air tube. This is a heck of a lot less work than making a trigger valve!
The paint nozzle is adjusted up or down as needed (more later) so a compression spring is slipped over the paint tube between the lock nut and the washer under the nozzle. This spring is not critical - its purpose is to keep the nozzle from turning when the gun is in use. I made the nozzles from round brass rod and cut the hexes with a spin index and the mill. If you don't have a mill, you could just knurl them on the lathe. Hex is better since the air nozzle should be properly tightened. I'll mention the paint nozzle again later.
The paint container is a glass jar. Choose one that is a size from some product you use quite a bit so you can aquire a number of empties. Take one jar and use the mild steel bottom plate as a quide to drill the lid. I keep some mineral spirits (paint thinner) in this jar and leave it on the gun when not painting - great for cleaning small parts. Replace the usual cardboard cap liner with one that will stand up to solvents. I made mine from some 1/16" thick teflon sheet. If you don't have any of this, cut one out of the flat, smooth side of a polyethylene bottle such as the ones motor oil comes in. The liner needs to be drilled with the guide. Use two 10-32 screws to fasten the lid and liner to the base of the gun so a jar can be screwed on as a paint container.
Now thread the brass vertical tube into the base, put on the lock nut loosely. Slip on the spring, washer and paint nozzle. Adjust the vertical tube so you can just get the paint nozzle on and off without hitting the air nozzle and tighten the lock nut in this position. You need to push a piece of plastic tube on to the bottom of the vertical tube. This tube should reach close to the bottom of the jar. I used teflon but polyethelyne would do. I'd be concered whether vinyl would stand up. You can always try it. I prefer a transparent material so I can see if it is properly cleaned.
The Al stand shown in the photos is too light to keep from being pulled over by an air hose. I plan to fasten a chunk of steel to the bottom to improve the stability.
Fill the jar about 2/3 full of mineral spirits, put it on the gun, hook up the air and turn the paint nozzle down as far as it goes. DON'T force it, damn it! Now trigger the air and screw the nozzle up until the spray just starts. Back off one flat on the paint nozzle and mark the position. I made a light punch mark on one of the flats with an automatic center punch.
Remember I said to use a jar that you could accumulate a set of? They make good paint storage containers (if you replace the cardboard lid liners) and you can set up to paint in a few seconds. You can use the drilling template you made to make one or more additional adapters to go on the gun for different size jars. With these and appropriate plastic pick up tubes, you could have, say, small, medium and large paint containers. There should be no problem handling at least up to a quart (a liter for some of us).
An Update to the Above (12/04/2007)
Email: Ted Edwards
Rev: 12/04/2007