Use a piece of sandpaper (paper side against it, abrasive out) on the piece you’re trying to mate to, and it’ll sand in the same profile.
You should use a dowel instead of your finger since it doesn’t deform too much as well. Basically you should use a sanding backing block, in the same profile of what you’re trying to sand it too. Flat block for a flat area, dowels for round, stuff like that.
