URO and sanding
I am not pro, but I have been playing.
First of use a guide coat before beating yourself to death chasing problems.
I recently got some hookit II 320 (real cheap) and was using it dry on the URO. That is the ticket. It sands fast and is high enough grit for final sanding on solid colors if you need to do that. The hookit II 320 lasts forever without buildup. I had been using some open cut 320 sheets for my primer.
I also have an older Dupont technical book from a class and they recommend URO at 4:1:3 for a sealer. I put on two coats as a sealer and did a light sanding with 600 wet (hey I have a lot of it). It sands out real nice and no scratches show through.
I also have been using the hookit II finishing film for color sanding and that is wonderful.