400, maybe go down to 320 if you need more cutting action. I'd probably start with green Scotchbrite like they use for scrubbing pots and pans. You're just trying to knock off loose material and little fibers that are sticking up, along with just rounding over the edge of the existing paint towards the exposed door surface so that the new liquid paint can flow up the "shoreline" of the existing paint. The whole sanding process should take 30 seconds, maybe less. What you don't want to do is sand away the embossed grain pattern in the "as manufactured" door surface. If you do, the atypical smoothness in that spot will show through the new paint and be noticeable. But most importantly, relax! It's just a door! Learn from, and be proud of, each repair you do!