I rarely use the vixen myself, mainly because of the higher crowns normally encountered, and they don't tend to match all that well across the panel
consistently. I'd rather use a flexible sander that conforms better, and as you suggest, less chance of metal removal. I agree that when you hit the metal, it's time to stop and do a bit more bumping or shrinking. I had read the pressure applied deal as a response on one of the metal shaping sites where someone raised concern of loss of panel thickness. IMO the vixen should not be used until the panel is to a state that the defects are difficult to see or feel, so unless you are looking for pure metal finishing with no filler, most can get by without it, exactly as you have shown. I rarely use the method myself, just thought I'd pass it along as food for thought if someone may want to try it.
On the giving lessons topic, your rear fender would have been a good video candidate for the shrinking disc, and your results would make anyone proud.. The few videos that I have done I try to keep less than 3-5 minutes, mainly due to file size before I convert the video. I can see a disc shrinking tutorial exceeding that unless one only focuses on a couple spots or extensively uses video editing. I seem to have better luck with hammers than trying to edit videos.
