So after a bit more of Will's attention today, the rear o/s quarter is probably the best it has been for 20+ years:
I found some pictures from back in 1991 and even then the edges of the arches were showing signs of corrosion!
I am stunned that this is just from welding/grinding and then a coat of etch primer - no filler yet. It really will need just the lightest skim - I did joke that with some paint and the decals, it wouldn't even need filler.
Any recommendations for a filler?
So, the next phase is the n/s rear quarter and sill, neither of which are as bad as this one was.