A major issue we run across all the time is the "bag of walnuts" multiple smaller dings, dents, lumps, and bumps surface of a panel due to poorly done dent removal, or just normal wear and tear a panel gets in decades of use. A shrinking disk is one of those "how did I ever do without this?" tools. Inexpensive, hard to screw up, and it works!
There are more expensive versions of the disk, but they do not work ANY better, so are not worth the extra money.
Here's a couple video demonstrations:
Shrinking Disc Video Demonstration - Wolfes Metal Fabrication

 

I have to keep reminding myself that it isn't as easy for Peter to get repop parts down there in Australia, and they are going to be much more expensive for him, so repair may be his only reasonable option.

 

I think by the looks of it , it was just a poorly welded patch that came loose and cracked the filler. The gravel pan looks pretty good from the backside of it.

 

If it was welded at all. The lip around the opening was not trimmed back to allow a proper butt weld to be done. I don't know what the tabs on the back side are for but they don't look welded on. It's issues and questions like this that necessitates the complete stripping of the panel of all paint and bondo to determine the exact condition. It's been real "interesting" to strip Gary's supposedly sandblasted and epoxy primed fenders to reveal the various work underneath. Everything from poorly done welds, cracks, questionable patches and doublers, brazing to bondo covered rust out and drill hole fillings, dents and hammer lumps. They looked fairly good at first glance in semi-gloss black. They are still actually pretty good compared to most F1 fenders I've seen.

 

That's a pretty good video. I have never seen a shrinker before. Is it just a piece of sheet metal trued up and set up for an angle grinder? I looked them up real quick and eastwood sells them for about $40 or so. Seems pretty simple. I might have to invest in one.

 

You could probably use a stainless steel pot lid although the one I have is flat. You need to be very careful you don's make contact with the spinning edge because it's like a meat slicer.

 

The shrinking disks sold by Wolfe on line http://www.wolfesmetalfabrication.com and on ebay are heavy (about 12 ga) tempered stainless with a depressed center and turned outer edge. Anyone who wants to make their own and can do it so it runs true, safe or can buy a 1/8" thick 9" SS fry pan cover and drill the center accurately for < 40.00 have at it. You'll also need a rubber sanding backing pad or one of the HMW plastic ones sold by Wolfe for the 9" disk.
Definitely worth the money!