Every modern guitar is sprayed with some type of finish. Some guitar companies use Nitrocellulose lacquer while other companies use polyurethane finish. There are pros and cons to both of these types of guitar finish, but they both have one thing in common. They both are sprayed onto the guitar body with a spray gun.
Spray guns use compressed air to spray the finish on the guitar. Basically, a spray gun is hooked up to an air compressor. The air compressor sends air through the gun to the gun nozzle. The nozzle focuses the stream of air into a fan shape. The finish is then injected into the air stream. Many spray guns work different ways, but this is how a common spray gun works. There are generally two types of spray guns: guns with cans on the top and the bottom. Spray guns with cans on the top of the gun claim to have a more even flow of finish because gravity pulls the finish to the gun. Spray guns with cans on the bottom of the gun have to suck the finish up to the gun. Both of these guns have their own problems. I use both of them for different applications.
There are also smaller airbrush spray guns. These smaller spray guns are used to spray designs and patterns on the guitar before it is fully finished. The most common design is obviously the sunburst. Sunbursts are easy to spray on guitars. Simply pick a few colors that you like. Them spray them on one at a time around the edges of the guitar each slightly overlapped the previous color. Allow the proper amount of time in between coats to dry. Then you can spray your finish coat on over top. And you have a sunburst.
If you are going to do any touch up work on guitars, you will need at least one spray gun. I have found that you don't always have to get the most expensive spray gun. There are some really expensive ones out there. Don't always buy the one that costs the most. It's not always the best. Look for guitar spray guns that are made predominately out of steel. You don't want one that is made out of plastic. Plastic guitar spray guns will just break on you. Take a look around and investigate before you go ahead and buy one. If you would like more information about how to finishing guitars, please see my how to finish a guitar article.