start at 180 grit after filler, then 320, then 600, lay a coat of primer surfacer down, then your paint. Also be sure to clean your valve cover better then shown, and MAKE SURE you wipe it down with a lint free cloth or rag and a actual paint grade cleaning agent, and i would also recommend an adhesion promoter if you really don't wanna prime. But 800 and 1000 grit is a waste of time and suited for wet sanding clear coat, not cutting primer. You could in all honesty do this job with 320 grit and 2 coats of a good primer surfacer and it would be smooth as glass. Keep in mind there is a pretty fair amount of heat and if you don't get this clean, your paint will fall off within a good hot summer day.
