Quoted from epeabs:
It's really very easy. I used my Dremel tool and a sandpaper drum like yours and it took no time at all and I had no issues. Just take your time and don't rush.
I'm in the process of hardtopping my Flash Gordon and I'd been trying to figure out how I was going to address this issue. Did you have any issues with the edges being rough, or did the Dremel/sandpaper drum method smooth it out pretty well?