I had a scrap sitting on the workbench that was big enough to wrap the drum. I think it was generic 80-grit. Harbor Fright, maybe?*What sort of sandpaper, and how did you secure it to the drum?
Anyway, I just trimmed it with a razor knife and stuck it on with a layer of very thin double-stick tape. Not the most graceful solution but it worked.
*I’ve mostly stopped buying generic sandpaper. Sure, it’s cheap but it doesn’t last, it sheds abrasive, the grading is erratic and it loads up almost instantly. Sometimes it ends up doing more harm than good.
There are times when it gets the job done (if the job isn’t especially critical) but it usually ends up being false economy.

