Polyester resin on bare, white styrofoam has always been a model makers worst nightmare…
Lookeee what I found!
Spray directly on polystyrene foam (Styrofoam).
Duratec StyroShield Primer is a polyester primer that is meant to be applied directly to polystyrene foam (also known as Styrofoam). > When cured, StyroShield forms a barrier that allows the use of conventional polyester laminating resins, gel coat, or other Duratec products. > StyroShield contains microspheres that insulate and protect polystyrene foam from the chemicals in polyester-based resins and gel coat. With StyroShield, low-cost polystyrene can be substituted for urethane foam in plug, mold, and pattern building applications. Spray StyroShield using an HVLP gun with a 2.5 mm or larger nozzle. Apply at least 20 mils of StyroShield to ensure complete coverage and surface protection. When in storage, the microspheres in StyroShield will float to the top and form a hard crust. Thorough mixing is required prior to use. When possible, store StyroShield Primer container upside down. Requires 2% MEKP.
At $134 a gallon, it’s not cheap, but the cost would be more than offset by the use of CHEAP styrofoam sheet. Not to mention not having to deal with urethane dust.
Fibre Glast also has 2’ x 4’ x '6 blocks of polyisocyanurate foam … if you have $1,000 that is.