Regular epoxy primer can be used as a seal coat right before painting.
It's the best sealer out there.
Mix the epoxy as normal and then reduce it 25 to 50%.
Check the tech sheet for your epoxy, most are the same.
You can spray a couple reduced coats and only have to wait about
an hour before you can paint over it.
I do that a lot because I have white and black epoxy and can mix
a shade that helps on coverage for different colors of paint.
It also helps eliminate scratches and greatly improoves adhesion.
I start sanding filler with 80 grit and finish with 150.
that's fine when going to cover with full strength 2k primer.
I sand primer with 600.
400 grit is usually enough to paint over, I recommend 600 just to
be safe, some colors like silver can show scratches if not enough
is applied. (400 is border line but most get away with it.)
Going any finer is not better or needed.
The primer in cans should be avoided, 1K products like that continue to
shrink long after applied. They rely on air drying, unlike 2K products that
use an activator and chemically cure.