to remove the black i would use 400 grit, dont use too much force. use sandpaper that can be used wet and do the process with either a bucket of water or in your basin with trickling water. the reason for the wet sanding is to keep the sanding consistency. you dont want the paint/crud distorting your sanding surface.
then take 1000grit when you got most of the black off and smooth out as much of the sanding abrasions again using wet sanding.
wash with plenty of water and some soap, use alcohol to remove any oil from the soap or your hands.
dry completely. use a tack towel and run a few passes to remove dust.
if you went through the primer, use adhesion promoter. do 3 light coats. dry it, run tack towel gently to rid any dust.
then take your gray primer with flex agent. do 3-4 coats until it's a smooth surface.
run 1500grit and get it super smooth. do another coat if necessary.
it's now ready for basecoat.
