You're wise to treat the rear of the disc first.

I had partially figured a 12" mirror a number of years ago, and I wasn't happy with the condition of the back side. It had suffered a few abrasions during the grinding and polishing stages, and I foolishly decided to clean it up, just using 400 grit, which leaves a nice very fine ground surface. Of course the disc back wasn't perfectly flat, so it took a fair number of operations to get it uniformly fine ground. The shock came when I placed the mirror on the test stand, and found it was all over the place. It had not even the resemblance of a sphere,

let alone a figure of revolution.

I was disgusted with myself, as if I had applied the two pennyworth of grey matter I do possess,. I would have left the back alone. After all, it was a fairly thin disc of plate glass.

The removal of glass that had occurred, was obviously sufficient to release some stress in the glass, resulting in the complete waste of time of all the figuring work I'd done previously.

That disc is still in a cupboard in my workshop. I lost the will to recover it.

Anyway, sorry to hear about your Car problems. Always a bad, and usually expensive time, and Christmas too makes it worse I guess.

Ron.

ive always said from the beginning that I have plenty of time to get it finished and getting the back flat is reallt important to getting a good overall finished mirror.

hope you will eventually get the motivation back to finish that 12" mirror as it will be good practice at rescuing a blank.

Mine is going very slowly too Crash. I first did centre through centre with the 11" tool although the back was going concave and the casting ripples around the edge weren't disappearing.

I've done some chordal grinding to bring the edge down with a smaller tool (actually the grinding stone).But now I need to work with chordal strokes nearer the centre as I'm getting a turned edge

You could probably have just switched to a full w-stroke with the full sized tool to get the casting ripples on the edge sorted.a smaller sub diameter tool is definately useful too