I used to do Auto Collision repair. It takes a lot of experience and skill to get to the point where you end up with straight looking car. Preparation is the key to a good paint job and it is the most difficult part. Any minor mistake will glare in your face once a glassy paint job is applied. As long as you don't have any major damage just a few dings, the paint shop should not charge that much more to do the prep themselves. They would probably like that better anyway.
