But is not compatible with expanded polystyrene, which is what Styrofoam is.

Duratec have Styroshield for this purpose. Doesn't work as well as you might hope, because the Styrofoam still has 0 strength so you tend to break through the coating all the time. So you're really best off using it to seal the foam, before using regular polyester/vinylester resins to build a fibreglass skin. But by the time you've done all that, you may as well have done the same thing with regular epoxy laminating resin.

Duratec Gray Surfacing Primer prepares composite plugs and patterns, as well as many wood products, with rapid coat build-up and easy sanding. ... wood, medium density fiberboard, brick, concrete, polyurethane foam, and most Epoxies. ... Starting from a correctly shaped and dimensionally stable plug,

Hello people 😁 .... any suggestions regarding the correct primer coat for a styrofoam pattern ?
Thanks stevr