Yo Flop, I have been in the sandblasting game for 35 years. so that is why I cringe at disc sanding panels.
That coupe paint looks pretty good.
I take it you do not have a sandblaster and I figure your old sand paper is clogging with paint and causing heat before you get to metal, as well as the sand marks in panel..
Try a heat gun and a sharp putty knife to melt paint off.
Then if you must, it will be a very light lick over with 120 grade.
Get a 2 inch wide putty knife and cut it back to about 1 1/2 inches LONG and round the edges off, so it is short and very stiff.
Heat the paint and you will know when it gets soft enough to peel off in sheets, way before any excess heat will warp panel.
Start on an angle with knife under paint and blow heat forward and keep going.
You would do that 1/4 panel in about 15 minutes.
Thick paint is best for heat, not so good with thin paint.
Some types of paint don't react too well to heat, then you must resort to stripper if you do not have a blaster.
If I have to strip a panel smooth, no blast profile for paint tooth, I run over with an orbital sander with worn out 36 grit.
Gives a perfect tooth for primer.
Here is one of my old videos. I will make a better one next time I have to blast and get dirty.
(apologies for work behind hand a bit, because I was holding cam with one hand while blasting and thinking the lens was getting peppered.
I will cover the cam with plastic next time for a better view).
