How to scuff up epoxy primer in order to spray the top coat?

.I would also still recommend looking at the tech sheet before just scuff and shooting. You do not want stuff peeling off later.
The PPG sheer says after the open window has closed to scuff, and reshoot with a coat of PPG DP40, 90 etc
I would assume it is a mechanical and chemical bond. Then a top coat of primers or paint.
Most manufactures have tech sheets I would advise you to read it for the products you are using