to prep the car there is a lot to be done...you got to sand it down first...then wash it and get the residue off, the you got to wet sand it so that you do not feel any rough spots....and then you have to get a special cleaner to clean off the finger prints and shit....trust me its a big pain in the ass..i preped mine before i had it painted..and sanding is a bitch casue you cant get the whole car with a DA sander...and even if you take it to them...as you said they dont do a good job all the time...they may touch the car and when it is painted it will fisheye....so if i was you id watch it when they do it or you will watch your money go down the drain...cause someone touched my front end and it fisheyed...

Happy Sanding