I had the same problem on my 65 Chevelle. I used a 7 inch Milwaukee grinder with abacking pad and Norton 40 grit blue disks. It worked great. I stripped the entire car to bare metal in about a day and one half. I then went over it with 80 grit on the grinder. DA'ed it and then sprayed with PPG epoxy primer. Pictures of this on my website.
http://webpages.charter.net/65chevelle/