If your going to bare metal. 80 grit on a DA is what you want to use. DONT PALM SAND IT. Seems like you need to look more into how to paint a car. Its nut just about abbrading it, its about leveling everything out and making it straight. Heres a write up of how I would paint a car:
Wash the car with soap and water and then clean it with wax and grease remover.
1. I started block sanding the car with 320 grit slowly featheredging all the scratches and nicks so that each coat of paint or "the halo" around the repair is at least 3/8ths of an inch wide. I did this in stages b/c I had the drive the car while I was doing to bodywork on it. lol
2. I then went over the car with my hand and circled all of the repair areas with a black marker so I dont miss any imperfections before priming.
3. Remove all the door handles, moudings, mirrors, lights, lock cylinders (a must), ground effects. The door mouldings, if they are in good shape, just leave them as they are very expensive (150 bucks each) and you dont want to mess em up.
4. You can then perform all the body work thats needed. If any metal work has been done, make sure you get the metal ground clean and epoxy prime it within 40 mintues of being ground clean. After the epoxy primer is cured, scuff the primer with a red scotch brite pad and take all the gloss off of it. Then start slingin the mud.
6. After the major body work has been performed, go over the whole car and make sure that you didnt miss any chips or dings and finish sand the body work to 320 grit. Using a damp cloth may help you rub the panel down easier and will allow you to feel any waves or imperfections.
5. When getting ready for primer. Mask the windows, Back mask the wheel wells and make paper wheel wells. I can't stand paint jobs that have color and primer in the wheel wells. Just make sure everything you dont want paint and primer to go is covered and correctly masked. I recommend taking the wheels off and setting the car up on jack stands and back masking allong the pinch weld and connects to the wheel well mask. Only do this if your in a NON downdraft booth. This creates a skirt around the car and prevents dust from the under body to get blown around and into your primer and paint.
6. Once the car is all masked up and ready to go do a thorough cleaning with wax and grease remover. When using it, its a wax on wax off method. Dont be affraid to clean it with the wax and grease remover 3 or 4 times. After its all clean, blow off the car and then follow up by tacking off the car with a tack rag to get all the fine dust off the car.
7. For the actual mixing spraying of primer, go by the paint makers recommendation for the products mix ratios. I recommend using PPG's DP series epoxy primers.
8. After its primed and all cured, spray a contrasting spray bomb color over the primer to act as a guide coat for when you block sand (wet or dry) the car. This will show you when all the low spots are removed and the surface is all level. I would start with P400 grit and work your way up to 600-800 on the whole car. Always sand parallel to the arch of the body line. Use a cross hatch method.
9. In my case, the primer has been exposed for a few months now and isnt the ideal surface for spraying over. So i am going to wet sand my car again, and re shoot an epoxy primer on my car, block it again to 600-800 grit to be ready for paint.
10. When shooting the car for paint, mask the car back up just like you did for priming. Taking extra care on the cleanliness of the tape job and keeping the spray environment just as clean as the car.
na j/k if youare wanting to bust off the paint to gety to the metal fast use liek a 250 grit. but plan on prepping it well before the paint.
