Here's how to use a spraygun to paint your projects and make them look great. A spraygun is a lot faster than a paintbrush. The result looks a lot better too. No brush marks!
Make everything around you look shipshape and professional. A wise lady once told me "A project isn't finished til there's a "finish" on it. Get it?"

Any object tends to look like crap after it's been outside for a while. Metal rusts. Paint flakes off. Wood discolors and starts to crack. And the project you spent so much time on is yet another eyesore to irk the normal people.

A spraygun is the magic wand that will solve your problem. It really is just like magic.
Paint a multicolored object all one color, and it suddenly looks like a real thing, not some crap stuck together. Paint it white, black, or grey to match the theme of your surroundings, and suddenly it vanishes. It's become a part of the surroundings. No one would think of making you get rid of it.

I just made this metal hand truck look like part of the solution.
A few minutes ago it definitely looked like part of the problem.

Stuff you'll need:

Safety Glasses
Hearing Protection
Rubber Gloves
Filter Mask with organic vapor (charcoal) cartridge
Spraygun
Air Hose
Pressure Regulator
Paint
Solvent
Angle Grinder with Cup Brush
Wire Brush
Sticks, rags, and something ugly, rusty, and dry to paint.

Here's the motion you'll be making with your spraygun:

Step 1: Safety Gear

The glasses keep paint droplets, flying dirt and wire bristles from hitting your eyes.
The mask keeps your lungs and brain from filling up with solvents and dirt.
The rubber gloves and long sleeves keep solvents from soaking in through your skin.
They also keep your hands skin-colored.
The jackhammer headphones preserve your hearing for future enjoyment. They also play mp3 lectures from the London School of Economics about how Instructables is awesome.

Step 2: Spraygun, Airhose, and Regulator

This is a "touchup" gun I got for $10 or so on sale from harborfreight. I also got some brass quick-detach fittings, some teflon tape to seal the threads, an airhose, a coiled hose, and a pressure regulator with a gauge.

The regulator has a water trap, which would help if the air here was wet. Some air compressors put a lot of water and/or oil into the air hoses. When that shoots into your paint it makes a mess. At an autobody shop they put an airfilter/watertrap right on the gun. The regulator goes near your gun. There's another dumb one back near the compressor, but by the time the air has flowed the half mile out here to the front line, that regulator is just another drag. You need this last-chance regulator out here near the gun to actually know and set the gun's pressure.

If you don't have a compressor you can use the airconditioner compressor on your car or the pump from an old refrigerator. Even if the chill is gone from the unit, it's still good enough to pump a lot of air. If your pump is wimpy it's not the end of the world. Get a smaller gun, bigger air tank, or wait for the pressure between shots.

William Martinez says that in his native El Salvador they skip all this nonsense. They spray cars with a hand pumped bug sprayer from a hardware store. You know, the thing from Tom and Jerry cartoons that looks a little like a bicycle pump with a can at the end.

I've also used hand-held airless sprayers. They're pretty good for dribbling paint on the pavement, taking apart, and giving to other people.

Step 3: Wire Brush the Metal

A hand brush is plenty for most jobs.
Don't go nuts, just make sure you've brushed everything that looks rusted or flaky.
The secret is that the most important stuff to knock off is the stuff that's easiest to remove.
I learned about wire-brushing rusty metal from my Dad. That's how they did it on the farm.

Step 4: Power Brush

For heavy rust or badly flaking paint, an angle grinder with a wire brush wheel makes the work go fast. I prefer the "Twisted Bristle Cup Brush" style seen here.

If you're spraying a car you'll probably use a sanding disk or an orbital sander with 200+ grit paper to scuff up the whole car. Wet-sanding by hand with 400 grit is just as good. Use the cup brush on the rough and rusty parts.

Step 5: Free Paint!

Paint is free unless you're in a poor country where waste is rare.
In a rich country everyone else already bought too much paint. Now they're worrying about toxic disposal fees and trying to give it away. Most towns have a paint reuse program.

At the SF transfer station ("the dump") they mix all the partially full paint cans together to make a single mud color. Then they try to give it away. I hope that works for them.

I scavenged these cans of oil-based paint and the can of acetone thinner.
If you care about mother earth you might hold out for "low-VOC" coatings.
Those are types of paint that emit less solvents to the atmosphere.
My understanding of scavenger economics says it's okay to use the old stuff if you're not buying it new. I seem to recall acetone is the least harmful of the polar solvents. Please correct my facts and/or philosophy.

There are two basic types of paint. Oil-based, which you thin with solvents, and Latex, which you could thin with water. But you aren't usually supposed to thin it. You can clean it out of your brushes with water. Latex is good on wood. It makes a semi-permeable water repellent film that lets the wood breathe a bit. Sort of like a gore-tex jacket for wood. Some expensive latex paint can even be put below the waterline on boats.

Oil-based paint is easier to spray because you can thin it out more without hurting the film quality. It's good on metal because the oily molecules repel water like a squashed duck.
Rust-O-Leum is oilbased. "Alkyd" means oil-based. "Enamel" used to always be oil-based. Now some latex paint has that word on the can. Humans are a bunch of sneaky bastards.

Step 6: The Real Opener

Don't use a screwdriver to open paint cans. It bites the lip of the lid and makes the seal leaky.
Then you'll get braindamage from breathing fumes and your paint will dry out in the can.
Get a real opener like these. Paint stores give them away for free.
The best ones look like this curved one. Your life just got better.

Step 7: Why Grey?

Grey #1!! Grey roolz!!!
Grey is the best of the primary colors, the others being black and white.
Why is that? Here's the comparison:

White has a lot of advantages. It adds light to your surroundings. It makes things look clean like a whitewashed Greek village, the kind none of us will ever visit.
Titanium dioxide is the most common white pigment. It's very opaque and conceals other old colors under it without a lot of coats. Any color that contains some white also covers very well.
Unfortunately white gets dirty really fast which doesn't look good. Solara's white decks look pretty dingy now. And the glare off the white(ish) decks is excessive on sunny days. A white thing that's scratched and rusted looks very scratched and rusted.
Bodymen ("panel beaters" in the commonwealth) know that white is the color that makes dented bodywork look the worst. The eye is very sensitive to the subtle shadings of an uneven white surface. But white cars are popular in hot sunny places because the sun doesn't heat the car so much.

Black hides rust pretty well when it starts rusting through. So you won't have to repaint so soon.
Black things get hot in the sun and dry out quickly if you're in a damp northern type place.
Carbon is the usual black pigment. Soot in various forms like "lampblack" are popular sources of the fine carbon particles needed to make a good pigment. Feel micro-good about sequestering tiny amounts of carbon!
Black surfaces can get dirty pretty easily since most dirt is a lighter color.
The glossyiness of black paint is a big deal. I don't know why. It's hard to match from one can to another. Expert readers insist that shiny black, not white, is the color that makes lumpy body work look the worst.

Grey hides bumps and dents better than black or white. It doesn't get hot in the sun but the glare doesn't blind you either. It hides dirt really well. It hides rust almost as well as black. You can mix the kind of grey you want from orphan cans of white and black.
Most importantly, grey things are invisible to normal people.
It psychologically blinds authority figures to your insane behavior. After you've painted all your stuff grey, it will appear as a unified, highly organized system. You will appear to be an indispensible member of a team.

Step 8: Thinner and Viscosity

I'm thinning my paint with acetone to make it spray properly. If it's too thick it will look splattery or speckly. If it's too runny it'll be easy to get "runs" or drips in your finish. I like acetone because it dries fast. And I seem to recall that it's not extremely harmful. But maybe braindamage has addled my memory of the USP manual.

I thin the paint to about 4 centipoise viscosity to make it work in my spraygun.
There's a lot of variety in sprayguns. Read about yours and do what they tell you. Or just goof around with it til it does what you want. Harborfreight has some cheap ones that are good enough to spray paint with.

The manual for your spraygun will tell you about the desired viscosity of the paint. Also a bunch of stuff about air pressures, flow rates, and adjustments. Possibly your spraygun will come with a "viscosity cup" which is a cup with a hole in the bottom for measuring viscosity.

Fill the cup. Count how many seconds it takes for the cup to drain. That's the viscosity in centipoise. Water is one centipoise and drains out of the cup in one second. Honey warmed in your armpit or cooking oil are thicker and slower. It drains out in 20 seconds and is 20 centipoise.

Some thinners won't work with some paints or varnishes. Don't test a new thinner on your show car. If you buy thinner at an autobody supply store they call it "reducer".They'll ask you what temperature and humidity you expect to do your painting in. It matters because car paint needs to sheet out and be all shiny. If you're in the SF bay area, regular lacquer thinner will work fine. Autobody stores do sell something called "paint thinner", but it's only for cleaning up the gun and other messes.

Step 9: The Stroke

The spraying motion is an art in itself. Move the gun at a constant velocity back and forth. Push the button after the gun is up to speed. Unpush the button before the end of the stroke. Overlap the strokes like shingles, wet on wet. Just like in this magnificent movie:

If you get runs it means too much paint or too much solvent in the paint.
Use lighter coats. Wait and paint on another light coat.
Go ahead and tell people you put 30 coats of hand-rubbed lacquer on your MacGuffin.

Step 10: Painted!

What a smooth grey thing!
It appears vital to our operations here on the base.

Next you'll want to clean your spraygun.
Instead I'm keeping mine wet by spraying something with it every day and keeping it out of the sun.
That sort of optimism is what causes sprayguns to fill with hardened paint.
Sort of like putting a dirty paintbrush in a plastic bag so you can clean it "later".
Read the cleaning instructions for your gun to see what the proper method is for that model.
I dump out the paint out of mine back into the can, wipe everything with a rag, then put some solvent in the gun, spray solvent, then dump that in the paint can to start thinning it for the future. I repeat that until I think my gun is clean. Spray the solvent at a surface to see how much pigment comes with it.
Some serious painters keep their gun in a bucket of solvent so the traces of paint left in it never harden.