this is a 92 rear bumper. it has some paint scratches two are deep some are just surface. its blue right now and i will need it to be black. The paint i will be using is the stuff from automotive touchup or paintscratch.
Fix the scratches and prime them. I wouldnt sand much coarser than 220 though.
-Get some glaze and fill the deep scratches
-Finish the repair with 320 and apply some primer to the repair areas.
-Sand the entire bumper incl primer with 600, followed by 800.
-Apply your color, followed by clear.
-Cut and Polish if necessary with 1000/1500/2000
There's no need to sand higher than 800 before color as you're putting something on top of it anyways, making the scratches slightly smaller wont do much in terms of changing the texture after paint, thats where spray technique, product and conditions really come into play.