It would depend on how deep the scratches were on your revolver as to what grit of sandpaper you would use. If the scratches are slight and not very deep, I wouldn't use sandpaper at all but use a buffing wheel instead with some rouge. Even if you use 600 grit sandpaper that will not get you the high polish shine you really want. And the better your bare metal preparation (in this case the buffed high gloss shine)....the better it will look when you re-blue it. It's kind of like body work on a car where the job will only look as good as your base preparation. Only more so in this case because bluing does not cover imperfections nearly as well as a thicker coat of auto paint since bluing has practically no thickness at all and the revolver's finish is wholly dependent upon your surface preparation before re-bluing.
If you have one or have a friend who has a bench grinder, you can quickly put a buffing wheel pad on it and using some rouge on the spinning wheel to do the job. Even if you have to use a file in some spots or heavier grit sandpaper to get all the scratches out, you would still be best served to use the buffing wheel as a final operation to mirror polish the bare metal before re-bluing. Actually before you touch it with sandpaper, try a buffing wheel first. Start with a heavier grit of rouge and then go to a smaller grit. You may find you don't even need any sandpaper to do the job. But again, that depends on the depth of the scratches.
Just be VERY careful of the flats on the barrel and other angled parts of the revolver that you do not wear them down and thus round them off with the buffing wheel.
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