before sparying a car with single stage. what should the final grit of sanding be. planned on spraying a sealer or primer/sealer first?

 

I would suggest spraying a sealer before you lay down color, that's just me. Typically I would finish with 600 or 800 grit wet, if sanding filler primer. If you are just spraying over top of old paint with no repairs you can use a scuff pad and sanding paste or sand it with 1000 grit.