Handsanding howto
A belt grinder is not a real option because the razor is hollow ground, so handsanding is your best bet imo.
If you really want to use a dremel, use flap wheels. Not drum wheels.
Removing a measurable amount of metal with a drum wheel is going to leave visible traces.
But the dremel flaps only come in low grit, so you'll have to do the high grit sanding by hand anyway unless perhaps if you use cutting and polishing compounds. But so far I haven't played with those.