Whether you want to paint your own car to save money, or just for the satisfaction of doing it yourself, you should know what you're up against before you begin.
Many amateurs make the mistake of believing you have to completely prime the whole car, which isn't necessarily the case. It isn't a bad idea, just not 100% necessary. If you're painting the whole car, a nice even coat of primer will give you a solid base to spray over, which will result in even coverage with less paint. Any area that has had bodywork, or where bare metal is exposed, must be primed before painting, but paint will adhere just as well to sanded paint.
There are many different types of primers ranging from general purpose primers to special primers for specific needs. Ask where you buy your paint to find out what will work best for your project. You might find yourself using an etching primer on bare metal, and a different primer for a full coat, or over filler. Ask at the paint store to make sure there are no compatibility issues.
Compatibility with paint is very important. Generally if you stick to one particular brand for everything from primer to clear coat, you should be fine, but it never hurts to ask just in case. You don't want to spend weeks getting the body perfect, and hundreds of dollars on paint, only to have to paint wrinkle because it reacted adversely to the primer.
Priming a car can be good practice for using a spray gun. At this stage you don't have to worry as much about getting a run, as they are much easier to sand out in primer. Also primer generally dries faster so you don't have to wait long to be able to sand out the problems. Some primers require a light coat over areas that have been sanded, others you can sand and apply paint directly over. Reading the instructions that come with the primers and paint products is very important at every step of this process.
Now that you have the car in primer, all sanded and cleaned, you're ready to start painting, right? Wrong! You've probably been doing the bodywork in your garage so move the car out and clean up.
Since most people don't have easy access to a fancy professional spray booth, this article is based on the assumption that you will be spraying your project at home in your garage. You have to treat your garage just as you would a paint booth because that's what you're using it for. Clean out all the dust-collecting items in the garage and then clean the garage itself. Sounds like common sense but many people overlook this important step and end up with a dust covered paint job and an angry wife because you got overspray on the Christmas decorations that were stored in the garage. Try your best to avoid both by doing a thorough clean-up. Wet the floor with water and move the car in.
Before spraying, make sure the car is clean. The cleaner the better. Use an air blower to ensure all gaps between panels are free from dust that might blow into the paint while spraying. Then use a good surface cleaner and clean cloths. Avoid touching the car from this point on as the natural oils in your skin can cause problems in the paint.
After an initial cleaning, mask the car if you haven't already done that before bringing it into the booth. Be sure to not leave any folds in the masking paper that dust or paint mist can gather in. Tape these up. Now do a second cleaning with cleaner and new cloths. Don't re-use the same ones because you may spread contaminants from the previous cleaning back onto the surface. Most surface cleaners are applied with one cloth and wiped off with another. When you're finished cleaning, go over the whole car with a tack cloth. It may look clean but the tack cloth will pick up the really tiny dust left behind. After tacking the whole car, give a light tack to the masking paper as well just in case there is any dust on it. You don't want that blowing into your paint job.
Now that the whole car and booth are ready, add the reducer, hardener, etc. to your paint following the manufacturer's instructions. I suggest taping a piece of cardboard to your booth's wall to use for test spraying. Adjust your air pressure according to the spraying directions, fill your gun using a funnel cone filter (or similar type of strainer), and test how the gun sprays on your cardboard. Once you're happy with how it sprays, start on the car.
Many beginners have difficulty keeping the air line off the paint. If you think this might be you, and your garage is big enough, have someone join you to watch the hose. Make sure you are both wearing respirators suitable for spraying paint.
I recommend starting spraying on the roof. Work your way down to the hood and work your way around the car. When you get back to where you began, the first coat is finished. Remember that the object is not to get full coverage in one coat. Don't be tempted to spray it on heavier to get better coverage. Multiple light coats are far better than one or two heavy coats. Usually three or four coats will achieve the coverage you desire. This is art, don't rush it!
Flash time is the time you wait between coats of paint. If the instructions say the flash time is 15 minutes, wait the full 15 minutes from the time you finished the first coat. Then start the second coat. Start each coat from the same place. That assures even flash time on each part of the car.