Easy, just start fine and move to rough then reverse when the result is achieved.
Now, as this is a pretty much unrepeatable exercise (material lost a considertion), be cautious at the start.
Maybe try a run with p2000 wet and dry and look at the results (I know this is uber cautious but what is the hurry at neglible cost). If you have not got close to a finish then go coarser, say p1600, and so on. Once you are close then reverse the process. Always go wet, be it with water or WD40.
There may well be short cuts but meh, what is the hurry! Not definative just time served.
