I have a nib tool but haven't had a chance to use it. Think you need a pretty steady hand so you don't gouge the rest of the surrounding paint. I usually wet sand them out if they aren't bad, but using 400 will be very tough to get the sand scratches out. I would think about starting with 2000 and if need be step down to 1500. With clear, I always start fine and work coarser if it's not doing the job. I have some compounds that claim they will buff out 1200 grit scratches but I'm not quite that optimistic without having some practice panel to try on first.