I really should say something here about the spray gun set up I am using, with some tips that I pickup up along the way. Most of you will NOT have a similar set-up to what I'm using.
In an ideal world, I'd have a sweet SATA or DeVilbis spray gun, with a 100 gallon compressor. Oh, and a well lit downdraft paint booth, and it would all be expensive. Part of the fun of this project is making do with what I could get my hands on. Interestingly enough, once I started to ask around I had two friends with pretty decent spray set-ups that were tried and tested. And a third friend with a big old compressor that would have let me use it if I just bought my own gun, which was a reasonable option. I still think buying all the nice stuff would be an investment for just one car. I even thought of renting out a local booth, but didn't look too hard since I had some good stuff readily available. Anyway...
The gun I am using is an Apollo HVLP "Atomizer" 7500T. From what I understand they invented the HVLP technology so they must have decent equipment. The gun I'm using is set-up as a siphon-feed pressure pot. This means the can with the paint is pressurized and sits under the nozzle. I could have purchased a gravity feed cup for this gun, but I didn't see the point. This gun allows you to fully invert the gun if needed (which turns out I did on a few spots at the bottom of the car where to pot would otherwise hit the ground.)
If you disagree, that's cool. I'm just glad I didn't have to tackle building something like either of these which would have increased my costs. One more reason to rent out a spray booth for final painting.
Here are two more things about gun set-up that you should spend some time thinking about, regardless of your set-up: Spray pattern...and cleaning.
Spray pattern: There are a lot of adjustments on the gun, and every time you change your fan width, or the media you are spraying, you've got to adjust the air pressure or material so your paint is atomizing correctly. There is a pretty good write-up on that here, quoting Brian Martin at Autobody101.com in his "Basics of basics of atomization". It starts about 25% down the page. This whole 'manual' from Burton is pretty useful info.
Cleaning: Don't forget that you need to clean out your gun when you are done spraying. None of the books or manuals talk about cleaning your gun. I followed the guide on the bottom of this page here: I put lacquer thinner in the cup and shake it until the excess paint is loosened up, then spray it through the gun. Then put in more fresh lacquer thinner, and spray it out the gun until it sprays clear. Then pull off all the metal parts that have paint on them and soak them in the cup for a bit, and then brush them all off with the 'pipe cleaner' thingies that came with the spray gun. I also bought a rattle-can of spray gun cleaner from the paint store that is good for getting into the nooks and crannies of the gun. (1/2 gallon lacquer thinner, $5; 1 can of pro spray gun cleaner, $5.)
Total costs for today: $10 for cleaning supplies.