400 is probably too coarse. 800 is ok, but 1200 might be better depending on how bad the run is. 30 min wait is about sufficient depending on weather and of coarse how bad the run is. Make sure you use very little to no pressure. Since you're rubbing back a run, make sure the paper is backed up with a block to keep it consistent. With fresh paint, I also suggest you use some soap to stop the paper from sticking.