This is a how to instructional on how to make a pretty good performing paint sprayer from two plastic bottles, a couple of straws, and some PVC pipe. You will also need a rubber band, two long screws, and possibly some electrical tape and/or duct tape.

Check out the video for the step by step instructions.

The paint sprayer will require some sort of compressed air. My design in this video requires a shop vac that can blow. One could also use an air compressor or some other sort of blower such as a mattress inflater.

I came up with this specific design while playing and experimenting in my garage. My design allows the paint to be aerosolized by the compressed air giving an even conical spray pattern. This is due to the position and orientation of the straws. I found that when the "sipper" straw is cut at a 45 degree angle and faces the oncoming compressed air, it produces the best results. The spray nozzle also must not extend too far past the sipper otherwise splatter will occur.

This paint sprayer can handle out of the can latex paint without needing any dilution with water. Just ensure the paint is properly mixed.

The great advantage of this design is that it is incredibly cheap to make, allows ease of refilling, and ease of cleanup (simply throw the bottle away and make a few more bottles with spray nozzles for next time).

This design isn't limited to spraying paint. One could spray varnish,
stain, oil, or fuel (flamethrower?)... as in a waste oil furnace configuration.

Step 1: Acquire Materials

This paint sprayer requires:

bubble tea straw (fat straw)
a normal drinking straw
two ~6 inch sections of 1 1/4" Plastic PVC pipe
two plastic bottles.1 Liter sizes seem to work best but it can be scaled up or down. The plastic bottle should have about a 1" diameter neck and should be flexible enough to squeeze.
Shop Vac
Two wood screws
Drill
Hot glue gun
Saw

Step 2: Construct Bottle Holder From First Pipe & Bottle

Measure Neck of Bottle and mark notch in pipe that is the same or slightly narrower. Make the notch 2" deep at least.
Cut notch in pipe
Place bottle neck over the pipe and measure the wider taper to cut the other notch in the pipe
Cut the second notch in pipe
Ensure the pipe fits snugly in the pipe

Step 3: Construct Paint Sprayer Out of Plastic Bottle

Drill a hole near the edge of the bottle cap so the drinking straw can be inserted snugly
Insert straw and cut at 45 degree angle
Trim straw so it only sticks into bottle less than 1 inch
Align 45 degree angle cut of the straw towards the inside of the bottle (the axis center)
Take the larger bubble tea straw and glue the 45 degree angle against the standard size drinking straw's 45 degree angle
Make the standard drinking straw extend into the bubble tea straw so it almost touches the top of the bubble tea straw

Step 4: Construct Shop Vac Adapter and Handle Out of 2nd Pipe and 2nd Plastic Bottle

Place 2nd pipe over the top of the 2nd plastic bottle to see where to cut off the plastic bottle
Cut top off of 2nd plastic bottle that is very slightly larger than the inner diameter of the pipe
Push the 2nd bottle top through the 2nd pipe so it sticks out the other end
Glue the 2nd bottle top to the 2nd pipe
Mark where the hole in the 2nd bottle cap will be by placing the 1st bottle (sprayer with straws) + holder against it and marking the diameter of the bubble tea straw
Drill hole in the 2nd Bottle cap
Ensure the bubble tea straw fits into the 2nd Bottle cap

If water like consistency paint is being sprayed, you can use the same
nozzle configuration but it may be more advantageous to use a coffee stirrer straw as the sipper and a regular sized straw as the air nozzle. Also if thin paint is used, you can extend the sipper straw so it reaches towards the bottom of the bottle and also drill an extra hole at the bottle cap for an air hole. This way the thin paint would be dispenses solely from the venturi effect and the paint sprayer would not have to be turned upside down and squeezed. Or yet another option is to omit the air hole, extend the straw to the bottom of the bottle and squeeze the bottle while it is still oriented towards the ground.

Step 5: Secure Pipes Together With Two Screws

Ensure you have a long enough wood screw
Drill a pilot hole for the two screws
Screw the two pipes together
Ensure Bottles can interface together (ensure the bubble tea straw fits into the bottle cap adapter)

Step 6: Secure Paint Bottle With a Rubber Band

When using the paint sprayer, the paint should be mixed and funneled
into the plastic bottle.
The cap/nozzle should be firmly screwed on
the bottle secured to the handle via a rubber band.

Step 7: Paint Sprayer Operating Procedure

To operate the paint sprayer, the paint sprayer is held bottle side
towards the ground. The shop vac is then turned on. When ready to paint, the paint sprayer is turned upside down so the bottle faces the sky. Then the bottle is lightly squeezed to dispense the paint in order to spray it. When done spraying, the spray gun must be turned bottle side down before the air is shut off in order to prevent paint spatter as the shop vac slows down.

An important reminder is to ensure that the paint sprayer is attached to the blowing side of the shop vac and not the suction side. Otherwise, the shop vac will suck all the paint out of the bottle and into the shop vac.