onbeachw320,
Okay, you may have blasted them to hell and back but the pictured you provided has some BROWN spots, where it should be gray tone.
If you're going to spray the centers with a color, you are, good to go.
Now let's talk about polishing/ sandind the outermost sections of the rim first.
How far you want to go is up to you. Do you want an appearance of chrome? mirror? or just something in between?
sealing your finished look with clearcoats is after. Temperatures and enviroment are another factor. with wheels, there is a wide selection of protectants.
To answer your other questions, Yes, absolutely, a primer. ( as discussed , tape off the parts you want to highlight.)
What Grit? on the parts of the rim you wat to Prime ? just clean and paint. spray , Spray , spray.
dakazman