i'd flat it back with 2000 wet and dry

buff with g3

then g10

and finish with black top :)

Hmm so do you leave the G3 on for a short time like a polish? Or not?

i.e. Would it be better to wash the whole car, then use G3 on one panel then wash it off, then g3 the next panel then wash off etc right round the whole car, and then polish and wax it?

nope it's got no protection it's an abrasive cutting compound that flats back the paint just buff over carefully as it's easy to burn through the paint