But yes, for a paint job to look good it is mostly prep. But prep also includes good body work, correctly sanded panels, tear down, etc. If the body work looks like crap, the paint will. If the body work looks good but is finished in like 320-400, you will see the scratches in the paint work.. you can buff them out temporarily but they will come back. Having everything clean is very important, obviously.
The art of it is getting a paint job that doesn't need any work once you are done spraying. but it is easier to smooth a slightly orange peel finish than to take out runs... as long as enough material is on there.