Knock on wood here but, I never prepare foamcore sheets.
I use the Insite Reveal, from Grimco, STRAIGHT OUT OF THE BOX on our FB700 (now three years old) and I have never had a static issue or one like you show here. We literally run the piss out of it every day and aside from wiping down my plastics (coro pvc, poly, and acrylic) with 99% alcohol, we do NO surface prep and have had the most outstanding results.
The first acrylic sign we printed is still up and looks great to this day. It gets western exposure every day from South Florida sunlight and looks snazzy still.
J