Originally, most paint guns were created as a "siphon feed," meaning their paint reservoir was below the spray nozzle. Today, with most states requiring HVLP (High Volume/Low Pressure) spray guns, almost all paint spray guns are gravity feed, with the paint reservoir above the spray nozzle. The trick to getting the best results from a gravity feed spray gun is to use the correct air pressure (per manufacturer's instructions), the correct nozzle (again as per manufacturer's instructions), paint of the right consistency and a constant motion of the spray gun while painting.
Run the paint through a paint filter to remove any impurities that might otherwise clog the tip of the gun.
Thin the paint as per the gun manufacturer's instructions. If you are using latex paint, thin with Flotrol and if you are using oil-based paint, thin with Penetrol. Never thin paint by more than 15 to 20 per cent.
Connect the spray gun to the air compressor and set the air pressure as per the gun's instructions.
Fill the gun's reservoir with paint. Turn on the air compressor and spray a light coat of paint onto a sheet of cardboard or scrap wood. Paint using a sweeping motion of the gun and do not put on too thick of a coat--it is always easier to add another thin coat of paint than it is to remove a coat. If the paint clots when coming out or if paint does not produce an even coat, then additional thinning may be required. If a line of paint appears at the top and/or bottom of the test swath, then your paint may be too thin. Add fresh paint to thicken.
Continue testing on cardboard or wood until you achieve a smooth and consistent coat. When painting, always use a smooth sweeping motion. Several light coats are better than one coat that is too thick.
Tape off anything near the painting site that you do not want painted. Using proper air pressure will minimise over-spray.
Always wear eye protection and a breathing mask.