EVA foam is a porous flexible surface. You don't need to scuff that. Sintra is a smooth, semi-rigid surface. Scuffing the surface creates microscopic scratches that the primer can settle into, causing it to grip onto the surface much tighter.
And yeah, Scotchbrite pad, they're sold both in the painting supplies section of the hardware store, and the dishwashing section of the grocery store. Chances are decent you've got one in your kitchen now, though you really want to use a fresh clean one.