preparation and application
It takes a lot of time to properly prepare a panel or whole car to paint. it takes skill and patience to master properly sanding a surface and feathering in different surfaces.
It takes a lot of practive patience and skill to properly spray a car. you need to learn proper gun control, speed, distance, overlap, equiptment selection, etc.....
If it was cheap and easy to do then quality paint jobs wouldnt cost thousands of dollars.