Some like a primer , some don’t … For years we applied the canvas to the foam with TB2 … DIRECT to the foam … No problems …
Gotta’ remember , nothing really sticks that well to foam as the foam surface always gives away ( fails ) first … The primers were originally used as a top coat over the glued and "sized “ to help fill in the weave and seams somewhat and make a smoother finish , before final painting …
Primer is Not really needed to stick the foam to the canvas !!!
Having said that , if you’re using EPS ( beaded foam) it “Might” be beneficial to give the beads a coat of primer to help consolidate the gluing surface

… We never did , but you might want to do that … XPS foam only really needs a good wiping down with alcohol to remove any oils left from manufacturing and “handling” before applying the canvas …
It’s the tight “Sock” effect that holds everything together …

The “Glue” just keeps your socks from falling down …
