There are quite a few painters who don't like to have someone else do the prep work - sometimes because they end up having to redo it. Sometimes because they're worried there may be problems with the paint down the road that aren't their fault.
That being said, you should be able to use bondo then sand it smooth to fill in the cracks.
Of course I'm generalizing from a single example here, but everyone does that. At least I do.
I wouldn't do a lot to the surface as far as primer goes. You can use a quality filler such as Bondo Ultimate. More expensive but , a lot easier to sand. Sand the repair area with 80 grit or less, and clean it very well. Mix the filler in correct amounts, and add small layers at a time. Sand with 120 grit and add layers until you have it perfect and then take it to the painter. I will say however most painters such as myself will not charge extra to fix small problems such as nicks, and don't put to much rattlecan primer over your metal parts because it will have to come back off. I will not honor my warranty unless I strip the panels to bare metal or plastic.
