You basically just need to get rid of all the shine of your paint work. once you havea matt finish on your surface it's ready for painting! use 800grit, if you want a reallllllly smooth finish use 1200 after the 800 too.
Not 100% sure if you are talking about using 1200 before or after top coat.
Before the primer you should use 600 wet and dry, after it is primed use 800 and to flat the top coat use 1200 upwards
using 1200 before painting will leave the surface too smooth and may lead to paint adhesion problems. Hence the reason why you flat back paint in the first place..
Hope this helps
Cheers