Most of the cars I work on already have 2-3 coats of paint. I personally use 36 grit discs on a DA, and gently sand down just until I see scratches to bare metal. It doesn't take long, as 36 grit media is just a little smaller than county road gravel :)
This usually leaves a coat of primer, with some scratches to the bare metal. This knocks the paint off in a hurry. Then I go back with 80 grit and take it down to the almost bare metal, again without really leaning on the sander, just letting the machine and the paper do the work. You can then finish up with a lighter grade (180? 220?) and begin building up. The DA (dual action) moves slow enough not to generate sufficient heat to warp panels on the '60's car (although I wouldn't do it on an '02!). A spinning disc moves the paper at a high speed at the edges... the larger the disc, the higher the speed and the greater likelyhood of making a gouge, hole, or warping the panel.
If you don't have sufficient air pressure to run your DA constantly, you might want to rent a bigger unit for a day... it'll save you days over having to wait for your compressor to catch up!
Thanks,
Steve Charette
Imperial Services
www.imperialservices.net
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