Getting back to my question...why are you sanding paint? I see now that you are doing a 'scuff and shoot' meaning you are going over the old paint. Of course the whole job is as good as what is underneath it. In this case I would clean and sand 320....enough to rough it up. For areas needing repair you will need to sand 80 grit in the area to hold the filler. You can then sand and feather the filler working your way from a coarse grit to 320. I would think a primer/sealer over top would be required before thinking of painting. You should see how good your repairs are after this. Blocking the repaired areas will show you the problems. At least that is what I would do. I don't want to come across as a pro because I am not but I have done a bit of work and a fair amount of research.