Denny, nice repair on the rear fender, I think using the shrinking disc works best on a crowned panel such as that fender to allow good access to the high spots, and your efforts show how well it works.
On the body file (vixen) you referred to, I would suggest to ease up on the pressure. You want to glide across the panel enough to remove the guide coat and show the high spots. It's not about removing metal.
