Again, "Sintra", not "Sinatra". Sinatra is a dead singer/actor.
You only need to paint whichever material you intend to have visible. If you are using a combination of Sintra and EVA foam, I presume this means you are gluing the EVA foam to the underside of the Sintra for the sake of comfort or adding bulk. This means the EVA foam isn't visible, so it doesn't need any paint or primer.
The only purpose of primer is to prepare a surface to be painted.
If you do want to use EVA foam as the outer surface of your armor, you generally apply something thick to smooth that porous surface, such as white glue with a touch of water, or multiple layers of gesso. Gesso is itself a primer, so after that you can paint your basecoat. With other substances like white glue, it's not a bad idea to apply a layer of primer.