Doing Bodywork on my own car need some assistance. Please

We are using the original paint as a base. We had taken the car to get work done on it and the guy took a majority of the dents out and spot primered where he worked. He did not sand the rest of the car. I want to know what sand paper I should start with before I start messing with filler if I do? And if I do what grits do I use to sand the filler with before the car is ready for a coat of epoxy primer to seal the old paint and filler in?

2. Do I sand the epoxy before I spray the 2k high build (If so what grits?)? Or just spray and start blocking the 2k?

3. What grit do I block the 2k with?

and then after blocking the 2k it's sealer and then paint correct?:prayer::prayer: