Sorry you are having trouble. Can I ask a few questions?
What kind of high build primer are you using?
Are you mixing it according to directions?
What temperature are you spraying at?
Is it spraying too dry?
What gun and tip do you have?
Are you waiting between coats?
It is not a good idea to add more reducer to get it to flow. This can lead to a too thick build and solvent popping.
Generally speaking, high build primer should sand easily and level quickly. Assuming your body work is ready for primer, and you are not trying to level deep dents that should have been already been hammered and dollied, or filled.
Epoxy is not meant to be used as a surfacer, so would be an expensive waste.
You should only need to block with 180 once using a guide coat to level.
Prime again with only a coat or two as needed to cover breakthroughs.
Sand with 320 and a guide coat to ensure the surface is flat.
You can spot prime if some areas need more leveling.
When you are satisfied that the body is where you want it, then what I do is tint the primer with the color the car will be and spray another coat over the whole car. This will leave a semi gloss surface that will show any flaws, high, low etc.
If it looks good, (which it should at this point), then just wet sand with 600 and DO NOT break through. if you break through, you need to apply primer in that area. When finished sanding you will have a smooth surface all one color ready for paint.
I do not use sealer nor do I recommend it, if you have a smooth unbroken surface, your top-coat will cover fine, and since its tinted it will cover quick.
I forgot to mention, you should clean the surface with wax and grease remover before each application of primer, and hopefully before you started the bodywork or applied anything! Follow the directions, don't just put it on and let it dry. Spray it on or use a clean white paper towel (bounty) to apply, then use another paper towel to wipe it off, then throw it away.
The idea being the contaminants float to the surface in the solvent then are wiped off with the paper towel. One sheet for each small section. I will go through a whole roll on a big car. Much cheaper to spend a dollar on a roll of paper towels, than to re-prime and sand!
Sorry for the long winded reply