Some solid color shades, yellow, red, etc, are semi-translucent and don't cover well. For better coverage choose a tinted primer closer to the color you are spraying. For brighter colors like viper red, I use a white sealer under those.
Having said that, needing 6-8 coats to cover sounds more like you're not getting enough material through the gun. Are you sure it's orange peel or perhaps the paint is spraying a bit dry, are the solvents matched to the temperature?
