Close up;
Basically sold the wrong stuff by the paint factors
The Tetrosyl stuff is rubber(?) synthetic based, whereas the 3M was tar based, so it did not set.
The Tetrosyl is fast drying and shrinks down onto the metal, so the non drying stuff underneath just let it shrink & move, hence the cracks.
   
Finally finished stripping the reacted underseal off, god what a crap job
 
But YAY!!!
new stonechip goes back on, mojo refound
   
All the stonechip is now on and dried, with no reaction this time

Laid a coat of primer on the stonechip & unmasked it all.

   

Another job was to weld in the towbar plate inserts .....probably would have been better to do this BEFORE I painted inside the boot so careful paint removal with the dremel and masking up to protect the paint from weld spatter saw them welded in. I touched in the boot paint with a small brush, flowed in surprisingly well actually, virtually invisible. Not that it matters as it will not be seen by anyone.

This is the end result, the inserts will just hold the towbar plate off the rear panel, so avoiding damage to the paint when it's fitted.

   
Progress slow at present as it's very dull trying to rub down all the tricky outcroppings on the rear of the chassis prior to paint. Also slightly reprofiling the rear wheel arch lip on the B post by the drivers door as I was not completely happy with it.
   
Reprimed the arch edges as sanding it down had broken through to the steel again;
   
And a view not normally seen, from underneath;
   
I've rubbed all the rear chassis rails down now, blinking fiddly they were too with all the brackets on them, but now got a coat of primer on them.
   

Side view of the rails here; not much time left to work on them now as off to hospital to get my knee fixed on wednesday, so no car work for 2 weeks ...............probably..........