I would strip the fenders to bare metal, then coat them with a magic marker or spray dykem.
2003_0101Image0002 by
eric2406, on Flickr
Then you can take a long block with 180 grit and it will physically show you where the damage is.
2003_0101Image0007 by
eric2406, on Flickr
With some skill and the proper tools, metal finishing can be faster and easier then spreading and sanding filler.
02dentbefore by
eric2406, on Flickr
After about 5 mins and nothing more then some simple hammer off dolly techniques.
Picture013 by
eric2406, on Flickr
Picture014 by
eric2406, on Flickr
Another fender that is metal finished and lightly polished with 180 grit to show the smoothness of the surface.
Picture052-1 by
eric2406, on Flickr
finishedfender by
eric2406, on Flickr