With your paint chosen, mixed, and thinned you can now start painting. (Don't forget to apply primer!)
If you are coming from spray paint you may know that the cans requires a certain temperature and humidity to paint correctly. While this is technically true for airbrushing, oil based paints at least can be used at really any temperature. The colder it is the longer it will take to dry. Still, I'd try to paint at a minimum of 55F. However, humidity should always be low. Never more than 70%, but the lower the better. I like to paint around 30-40%.
This guide assumes your airbrush is similar to the Paasche H Model.
There are a few different needle sizes; 1, 3, and 5. The smaller the number the thinner the needle and in turn the smaller the spray. In my experience a number 3 is a great size.
Assemble your airbrush. Begin by putting the needle through the mount on the airbrush. Slide the needle tip over it and into the air cap. Screw the needle and tip about half-way together. Unscrewing and screwing these parts together adjusts the spray width. The more screwed together they are the smaller the spray.
Tighten the set screw to hold the needle securely in place. (My set screw is very long because I went to Ace Hardware and bought a longer screw so I can hold the needle better)
Needle Tip and Needle
Needle through the holder
Needle tip around the needle and though the air cap
Tighten the set screw
Insert a jar into the rear of the needle. Make sure it is securly inserted.
Turn on your air compressor, hold the airbrush similarly to a pencil, and begin testing the spray on a scrap surface.
Adjust the PSI of your air compressor and the size of your spray as needed. I tend to set my air compressor anywhere from 25 PSI to 45 PSI. Depends on the paint thickness, size of the spray, and how small I want the paint droplets.
While testing the paint also play with painting techniques. With spray paint it's best to spray in lines, changing direction while not painting the object. The same method can be applied to airbrushing, or other methods can be used. Due to less amount of paint compared to spray cans and the added amount of control, I prefer to paint in circles. It seems to cover better and faster, and without running.
Painting in Circles
Painting in Lines
When you are comfortable with your spray size and paint style, go ahead and start painting your piece. If you are coming from spray paint like I was, you will need to remember that airbrush paint layers are thin. With spray paint you could get away with two layers, maybe one. With airbrushing one layer, if it even covers everything, can rub off easily. I've found myself putting three or more layers on after the piece is covered evenly. Waiting until the paint is fully dry, as a opposed to waiting until it is dry to the touch, to put on another layer also helps make the paint layers stronger and thicker.
Dry time varies because of a few factors. Temperature, amount of thinner, thickness of layers, and type of paint all play a part. In my experience a part can be dry to the touch minutes after painting if the sun is shining on the paint and the temperature is around 70F. If colder the paint could take 10 minutes to dry to the touch. For a fully dry piece I typically wait an hour or two.
Paint collects in the needle and the needle tip so swapping jars won't quickly change colors. To properly change colors remove the current jar and attach a jar filled with thinner. Wad up a paper towel and cover the needle tip with it. Trigger the airbrush making air blow down into the thinner filled jar. This forces all the paint back out. Remove the towel and trigger the airbrush again causing thinner to flow through the needle and taking any paint with it.
Never store your airbrush without cleaning it. The paint will dry and either ruid the needle or make it very difficult to clean.
Fill a jar with thinner, unscrew the needle and tip, and place them in the thinner. Swirl them around a little to wash off the big drops of paint. Let the parts sit in the thinner for a few hours to fully clean them. If you plan on painting again soon you can leave them in the thinner to ensure they stay clean. If not, remove them and store them somewhere that they will not get damaged.
Rarely will you finish an entire jar of paint, so you must store them properly. The jars have two openings and through that the paint can dry while it sits. To stop this, slide a rubber nub over the metal spout and cover the small hole on the top with tape (anything but scotch as it doesn't seal very well). With the two holes covered, air can not escape so the paint can not dry.