When rubbing 2k primer, you will need to rub it with something more coase 1st to flatten it off and remove imperfections, this is where the guide coat comes into it, then use something finer to remove the scratches.
Thiers many ways to do this
For 2k Solid either block by hand with 240 dry or 400 wet, or use a sander with a soft pad with 320, then 600 to remove scratches. Only basecoat requires 800 wet to finish it off as base coat shows up scratches,
But 800 on its own as you said will not be coarse enough to flatten it off properly, is also very time consuming.
Altho solid white hides everything!